The DSi XL is just the DSi with huge screen. The original DS's is rather small to begin with, so this would make the screen HIGHLY pixelated.
The PSPgo is the PSP 2000 series with a smaller screen and no UMD slot. Even if you would download games to it's internal flash memory, you could only have 10 games on it. That's without counting in music, videos, photos, and extras, making that totally useless. Also, with no UMD slot and only accepted Micro Memory Sticks, it means all your previous games and sticks are useless. This makes bakwords compatibly out of the question unless you want to re-buy ALL your games. Even if you did, you could only have 10 at a time, unless you re-buy all those Memory Sticks. I suppose, if you used the firmware 6.10 exploit, it would make a nice portable emulation device; however, Sony has been trying to fix this, so it might no longer be possible in the future.
When compared the DSi XL is the better system just for it's ability to use it's originator's features (excluding the GBA slot). ~STBE