Well lets do side-side comparison.
HARDWARE
Portability: N900
It is smaller than Pandora, eliminates the necessity to carry a phone, needs to be accompanied with a BT controller (Wiimote) for proper gaming experience
Form-factor: N900 if there has to be a winner
It is tough to decide and depends on the individual's perspective, but the Slide design is well-appreciated for its quick use over the protective clamshell design which may be embarrassing for some people to use in public.
Screen: Pandora, if there has to be a winner.
Both offer high pixels (800x480) TFT? LCD and a resistive touch input however the Pandora is larger at 4.3" versus 3.5"
Keyboard: Tie
I haven't personally used the N900's keyboard and admit the 3-row layout is a let down but many reviews found it satisfying enough. The Pandora isn't out yet so obviously haven't tried that either, I believe it has a better layout but not fond of the shape and feel of the "square bubble" buttons.
Gamepad: Pandora
The Pandora has a physical gamepad so BT gamepads aren't necessary. The N900 only has a physical keyboard that can be applied as a gamepad. The experience might be enhanced with a
Game Gripper over the keyboard. The N900 can beautifully use BT gamepads like Wiimote or Sixaxis, this means the N900 must be placed on a surface to allow both hands access to the BT controller vastly affecting its portability and functionality. The best solution would be a USB-gamepad but it has no USB host (!!!) so this is impossible.
Camera: N900, no-brainer!
A Dual-flash 5MP Carl Zeiss isn't something to throw your DSLR away but I probably would throw my digital camera away for it as it can WVGA video at 25fps. There is 2nd VGA camera for video-call or like an online webcam for Skype/MSN. Pandora has no built-in and would need a USB-Webcam.
Memory: Pandora, if there has to be a winner.
N900 has 32GB inbuilt supports 16GB microSD. Pandora has none but supports 2x 32GB SD card. I prefer inbuilt+SD over 2x SD since it simplifies matters but Pandora can have a slightly more memory total 64GB vs 48GB.
Battery Life: Pandora, if there has to be a winner.
Pandora should have slightly better efficiency due to lack of 3G antenna (which can be switched off on the N900!) but it also has a larger 4000mAh battery than N900's 1320mAh. Be on the look out for larger battery accessories for N900 that may level the field.
Connectivity: N900, no-brainer.
Both have Bluetooth, Wifi.
N900 has extra Quadband GSM, Triband 3G, EDGE, A-GPS, infrared and an FM transmitter to sweeten the deal.
Other functions: N900 narrowly surpasses the Pandora due to G-sensor though lacking USB-host
N900 has a TV-out, digital compass, G-Sensor and miniUSB. Pandora has a TV-out and full-fledged USB-2.0
The Brains/Processor: Tie
Both systems have the "same" processor (600MHz TI OMAP3530 -> overclock to a fast and stable 800MHz), same graphics chip (SGX530), same RAM (DDR 256MB). HOWEVER the N900 can have 1GB RAM (virtual) due to the inclusion of the internal storage, which MAY make some games or emulators run a little more smoothly (thinking N64?).
SOFTWARE
Operating System: N900
Pandora runs on a customized Angstrong-Linux (kernel 2.6). This means that virtually all Linux programs including their desktop companions are supported. The only exception is for programs with 3D rendering which need recompiling to OGLS from OGL and could mean re-coding everything depending on the program. It is the same story in N900's case although they have the benefit of a large company to smooth the rough edges. The N900 is dual-boot happy and currently does Android "blissfully" much to many other smartphone's envy. The OS is rumored to be officially upgraded to MeeGo the new All-in-one solution to Mobile computing which should allow a greater (closed source) 3rd party application support to N900 than the Pandora.
Multimedia: Tie
N900 has full-flash support plus other formats. Pandora has similar. But both can support the same formats unsupported "out of the box" by 3rd party programs.
Games: Tie!!!
Pandora will automatically adopt most (if not all) games working on the Wiz, Dingoo etc. This includes many emulators (N64, PSX, GBA, Amiga etc) and Open-licensed games (QIII, Doom etc). The Pandora has interested no mobile gaming company (like Gamesoft, EA etc) yet. However, eventually Pandora SHOULD pick-up many games adopted from the Linux front (Super Tux anyone?).
On the other hand, the N900 has the Ovi-store. Currently their is very little in terms of games. Due to the similarity between both devices, the N900 can "steal" emulators and games running on the Pandora with little recompiling needed. So in fact, the N900 POTENTIALLY has the same library of games as the Pandora if not more (Ovi store). Note: games designed for the Pandora will probably become available on the N900 sometime after it has become adopted on the Pandora.
Community Support: Tie
N900 is backed by the biggest mobile producer in the world: Nokia. Hence, it is justifiable to say they are more capable as they have the money and manpower to enhance its software. But being an Open platform the N900 is currently attracting devs from a vast range of fields therefore it has a community and many fan-devs over at Maemo/Mer which is likely to become the "MeeGo" community. It is aimed at bringing full desktop computing experience to a phone and also fixing bugs and polishing the interface (eg Dual booting Android). The Pandora is the successor to the Wiz and has its community backing it up. It seems more gaming biased and they also actively strive to polish software and add new 3rd party application support. All depends on the way you view each community.
WRAP UP
N900 is a much more polished device with more functions, better OS and a community. The Pandora is the underdog, it doesn't have all the functions of the N900 or its sex-appeal but it isn't obsolete. The Pandora gives you a tried-and-tested gamepad plus the keyboard, a bigger screen that makes that big difference for some games (bubblebreaker anyone?), a trully Open OS and a strong online community. For most nerds the N900 is the iPhone killer and the must-have-gadget but for the gaming enthusiast the latter is slightly more appealing.
PERSONAL INPUT
I would make the choice between an N900 + BT controller (Xbox, PS3, Wiimote) vs the Pandora + cheap/used smartphone (Touch Pro).
Now if we look at the specs it is dead even with HTC Touch Pro, allows all of the N900's functions (except the unimportant infrared & FM transmitter) to the equation.
Now if we put N900 + Wiimote we would get "equal" portability to Pandora + Touch Pro 2 when we want gaming console and phone.
If we combine N900 + PS3 controller it is less portable than Pandora + TP but allows more comfortable gaming input.
Now if we look at the costs (in Aust.) it would look like this:
(N900 unlocked) AU$700 + (Wiimote) AU$90 = AU$790
(Pandora) AU$360 + (HTC Touch Pro) AU$250 = AU$610
Would you upgrade to the N900 team for AU$180 ?
Considering N900's bright future with MeeGo and Open game titles from the Pandora, I would if I could afford it
and even if it means I
have to grow an extra 2 arms and hands like Ganesh have to wait for someone to create a clip on accessory with PS2-style gamepad that BT commands to the N900.
ed: straighten somethings out: N900 does have a digital compass
afaik but the software doesn't use it yet
And the samething with MMS, it can do it but software support is not ready for it (which isn't a suprise as everyone on Maemo is shifting attention to MeeGo)
You are posting into a Pandora thread so I wouldn't be surprised for biased comments but I have tried to keep neutral on the terms and to keep my opinion separate to the comparison (ie under "Personal Input").
h34r: 'd!