Here's my opinions on your options:
DS - Never homebrewed my DS, so I can't advise there.
Dingoo A320 - Nice and cheap, at around $80. Good build quality from what I've heard, comes preloaded with emulators. To get more functionality/better emulators you'll have to install Dingux though, which I've heard comes with the risk of corrupting... something in it, I'm afraid I don't quite remember off the top of my head. Some reputable dealers are www.dingoo-digital-usa.com or www.dingoo.co.uk, and you'll want to choose either depending on where you live (DDU for USA, *.co.uk fo UK). Also, you may find in your searching if you look up other retro handhelds one sold by ThinkGeek. This is actually a Dingoo A320 as well, though a bit of a ripoff at $100.
GP2X: Can't comment much on it, but I've heard good things about this one too. It comes in two models, F-100 & F-200, one having a joystick and the other a D-Pad. I've heard the joystick is somewhat poor. Might be cheap to get one of them. It has a very large software library.
Wiz: I've heard complaints about the buttons here; mostly that the D-Pad and ABYX buttons are too close/ too small. It's very pocketable. New Wizzes are being manufactured and sold in $125 packages with Wi-Fi adapters and TV-Out(?) included. OLED screen is amazing, but some have ended up damaging the screen or otherwise ending up with ugly dead pixels. I'd say to consider this possibilty if considering a Wiz; it may not end up having the longevity of the other consoles, and since GPH is due to stop manufacturing them soon, you may not be able to get a replacement easily. Software library is pretty decent, but not as large as the GP2X (Though GINGE allows you to use GP2X software on the Wiz, to varying degrees of success.).
Caanoo: Good build quality, at first there was some animosity about the analog joystick, but since it's come out people seem to be getting along fine with it. Some believe it's GPH's best console to date. It even has dedicated MAME buttons and can be held vertically for arcade games. I'm getting the Caanoo myself. It's a good compromise between Wiz and Caanoo. Software Library is small, but slowly and surely getting larger (right now there are GameBoy, CPC, Mame, and SNES emulators, as well as PicoDrive, a great emulator encompassing almost all of the Sega systems). It'll cost you $140 for the console itself from www.dingoo-digital-usa.com (['teh smallest amount of $$$'? The spamtags are annoying] I've seen, at least for US). PlayAsia and others sell it as well, but it's somewhat more expensive from them.
Pandora: Simply awesome. Pretty much everyone that has one loves it. Emulates even N64 smoothly from what I've heard. There are only two real downsides I see with it; it's the most expensive, and least available. Unless you get yours on Ebay (where it will be much more expensive), you'll probably be waiting a couple months to get yours at this point. Also, because less people have them at this point, the dev pool is smaller and there are less apps/games for it than Wiz (at least for now).
Out of all of them, I'd recommend the Caanoo; but I'd say get a Pandora if you are patient and/or have the money as well.
