Some of my votes and why:
Screen size: 480x272 for both. There are some benefits for 320x240, but basically, I feel more pixels are better. Widescreen might not be very helpful for porting most PC applications but there is a lot of new content where it certainly doesn't hurt - for sidescrollers, horizontal shooters, and anything that has a lot of user interface, including shells. I agree that they're better for web browsers, and even if the popular opinion is that no one uses web browsers on these devices there's obviously enough of a demand for them that it matters. You know, there's a lot of web browsing you can do that doesn't involve a ton of user input. Like reading forums, for instance...
Of course this adds to the price, which is bad for the mini, but I agree with the sentiment that the resolution should be the same for both. If they're different then at best you'll probably see "widescreen extended" modes that add a little bit to the sides.
I think that the complaints are overstated. PSP uses 480x272 and I rarely, if ever, see anyone complain about it, even though primarily non-widescreen is used for homebrew. Sure, most emulators aren't widescreen, but on the other hand they're not 4:3 either, and no one seems to care about running their 256x224 NES/SNES emulators at 320x240. And GBA, which is just about the holy grail of portable emulation, works better on 480x272 (GBA's resolution is 240x160), IF the handheld can provide decent hardware accelerated bilinear filtering. Which is just a serious win in a lot of areas.
Everything else is/has been moving to widescreen, apparently it's the "future" (or maybe the present now?), both for games, TV/movies, and desktop stuff, so you'd figure people were serious about it.
Input: Analog for both. In retrospect I think a very good d-pad might be better, but I've seen so many awful d-pads lately that I'm losing hope in them, especially when done by a non-established company. Basically, the way I see it, a great analog stick will be good to great in 90% of games, and just bearable in the other 10%. A great dpad will be great in maybe 25% of the games, good in 50% of the games, and okay in the other 25%. In other words, it'll cut it for just about everything, but it'll rarely excel like an analog stick will in certain situations. That's why it might be better for a general purpose only-option. On the other hand, a crappy d-pad will make your life miserable, especially if it can't handle diagonal input at all like PSP's. Circular d-pads are more likely to handle diagonals well, but they tend to be less durable (the best ones I've used have broken on me a few times...)
Batteries: Li-on built in. I understand some people think rechargeable NiMH batteries are superior, but I don't really see any strong evidence there (but we'd need a comparison of the same device using both). No matter what rechargeable batteries are a hassle. Sure, not having a builtin rechargeable battery makes the base unit cost more, but not having it means you either have to buy rechargeables AND a charger, which likely WILL run you more than that cost would have if they're of any quality, or have to constantly buy one time batteries, which is infinitely less convenient and more expensive in the long run.
RAM: 128MB for normal, 64MB for mini.. gotta differentiate the two somehow. RAM is somewhat adaptable for at least some applications; 64MB is plenty for most (given the level of CPU power), so the extra RAM is good for scaling (running more stuff at the same time, dealing with larger things)... furthermore, since the platform has an MMU and Linux there's virtual memory, so requiring 128MB would at least not cut out mini users entirely. Yes, virtual memory can SOMETIMES make even a single program that needs all that RAM still useable, depending on various things!
Brightness/volume: Hardware. A dial for volume would be great. I miss dials. If not at least include easy to push buttons (urgh, not like PSP's please). Although IF buttons are used couldn't software work in conjunction with hardware?
Now, about the little side argument here... I'm the main developer of MZX (or at least have been for some years) and the one who ported it to PSP (although unreleased as of the moment), and yes, it does much better there than on GP2X. Of course a huge part of this is because PSP has very simple to use bilinear filtering. I don't know if GP2X does, but SDL sure isn't using it, yet is using hardware scaling. So I imagine if it has it it's not trivial to utilize. At 320x240 it, a 640x350 program, looks terrible without filtering. And yes, people have mentioned that text is hard to read on the very thread you source to saying they haven't (third post down), and much of those comments are about how to improve the scaling! But, even with bilinear filtering, text would still be kinda hard to read. When you DO have filtering the more resolution you have, the better, period. Aspect ratio might get messed up, but sometimes that's a loss that people are willing to take for more retained detail. Even if you're porting a 640x480 app (like JaqMs was saying) 480x272 is still superior to 320x240 in many cases if you have filtering. That's my opinion, anyway.
Shikaku, really, do you have to be so incredibly arrogant and disrespectful in your responses here? I can see JaqMs doesn't have a very good reputation here (I understand, we've had our dealings with him on our board too ;P) but does someone really have to have extensive programming experience to have a worthwhile opinion on the usefulness of a resolution/aspect ratio?
And geez, lay off MZX
Yes you did insult it. No, I don't blame you for having zero interest in it. Yes it's a niche program, but that doesn't make it and programs like it irrelevant just because you don't like them. And it's not an RPG maker or an adventure game maker (text or otherwise), almost all genres of games have been made in it, with a pretty decent distribution amongst a few of them. There also happens to be a small but pretty dedicated and close knit community surrounding the program, which also didn't deserve your attack against them (before you say "where did I say that?" I'm referring to this: "except with horrible games along with the horrible communities."). I've been in communities like that and I've been in communities like this and neither deserve those kinds of generalizations off the bat from people who haven't experienced the particular community in question first hand.
Also, before you go slagging MZX's lame scripting language and making fun of its users, calling them non programmers, bear in mind that a lot of capable programmers got their start with it. Myself included.
Not to be too pissy about all this, but I've just about had it with all of the over-conceited programmers lurking on the internet who think they're just about the greatest thing ever, and if you respond to me like that I'm totally going to go at you without holding back >_>