Personally, I would prefer the NDSL because first, it truly is 'portable'. It's smaller than the PSP therefore far easy to carry around. Also, because of it's clamshell design, the interior is likely to be damaged compare to the PSP which is more damage prone. Sure there are protections for both parties, which means more size and weight.
(NDSL winning so far)

So now let's move on the graphics. Obviously, PSP has better graphics, but, think about, since when does better graphics = better games? Isn't the whole point of getting a gaming console is having fun with good games? Again, there are more fun games that is playable on the NDSL (Combining the old classic GBA games) compared to the PSP. Also, Sony's UMD disc isn't a very nice thing to carry around when you travel.

Now, to R4 vs the Memory stick. The Memory stick and the MicroSD card is about the same price, but, a DS's 'Homebrew games' are on average 50~60 MB. Which means, you could have nearly 20 games on a 1 GB MicroSD card. The PSP's 'homebrew games' are on average near 10 times that amount, you'll be lucky to shove 3 games into one card.

So if you ask me, the NDSL is no doubt better than the PSP overall has a portable gaming device. Sony's decision to make a 'all-in-one portable gaming system' is probably the PSP's biggest flaw. So if you always on the rush who need a quick shot of gaming, the NDSL is the way to go.