This is totally from my own experience and not from any technical analysis:
The original 3DS is a good degree stronger than the PSP, but not massively. Although, this probably isn't as obvious visually as the 3DS is rendering a second screen and, for most games, 3D. The New 3DS is even stronger and can run Xenoblade Chronicles (one of the most graphically impressive Wii titles) compromising only on texture quality. A lot less was compromised than when Persona 3 was ported to the PSP.
It's worth noting thought that the graphical quality of most games is often down to budget and effort. For example, FFVII Crisis Core on the PSP looks far better and more polished than what I've seen so far for Final Fantasy Explorers, despite the latter being on more powerful hardware.
However, the 3DS' screens are only 240p, while the PSP's is 480p. This allows the 3DS to have more graphical effects that it otherwise would (arguably making the aforementioned Xenoblade Chronicles port possible). Despite this, I really hope Nintendo finally get with the times resolution wise with the NX.
The 3DS and especially the New 3DS probably have much higher RAM than the PSP. I'm basing this off functionality while 'presssing the Home and Playstation buttons. On the 3DS, the game is fully paused and one can read messages, check the online status of friends, etc. Streetpass also works while playing games. This isn't capable on the PSP, where the game doesn't even pause itself iirc.
Battery life wise, my New 3DS is longer, but not considerably than my PSP 3000.
It'll be quite interesting to see whether my experiences and thoughts are actually backed up by fact.
Xenoblades biggest texture problem was the rom size. The game was compressed nearly a GB over the original (Japanese game, with only the single voice track) to fit on the 4Gb ds cart.
The other problem was the the textures for the wii game were designed for a 640x480 resolution and the 3ds screen is 800x240, but split across 2 eyes, which gives you 400x240 (Going 2d literally halves your texture detail, always put 3d on, even if you dont like the effect, just enough so the second image 'activates' you can literally SEE the texture information double) The problem with this is, when you have textures that just have that many more more texels than the screen space they are going to be appearing on has pixels, in particular, when you are dealing with a res as low as the 3ds screen (And lets face it, Xenoblades textures werent exactly high res, and YES aliasing is definately related to this but not what Im exclusively talking about specifically here).... It just washes out so much detail, and then when you try filtering it for that tiny resolution, it just gets worse. And then you consider the go to solution of mip maps were generated for the wii version and transferred on over seemingly untouched, they also seem to have too many texels for their pixel screen space, they often don't jive AT ALL and you just get this giant blocky mosaic from certain distances and angles. Ugly Ugly Ugly.
That being said, I dont mean to come down hard on monster games, it is a phenominal port, damn near miraculous. The only way they could have avoided these issues is if they remade all the textures for the 3ds, and that game has a LOT of textures, its kinda big (Just ask the dolphin hd texture pack guys). Just compressing them to fit on the tiny cart is the only realistic practical thing you can really expect without putting in the money for a full blown remake.
Im pretty sure most devs who have gotten good looking games out of the 3ds by now, have learned to take the cereal box approach to texturing. You know, the little disclaimer 'Textures enlarged for detail' They still do that on cereal boxes right?
I used to have this wonderful article on this subject, much clearer than my ramblings, and it actually had pictures showing at what point continuing to increase the texture resolution would not just stop showing improvement, but actually begin looking worse... But its become impossible to find, not surprising, as its really not an issue in today's world of ludicrous screen resolutions.... Just little 3ds.
I am so easily distracted by tangents.
