I have a PSP and a DS...
DS is great for games. Ruiner, the Dual Screens aren't uncomfortable at all, and gameplay DOES involve both screens. However, it isn't hard to play. You use the pen and the bottom screen to move your character, and you look top screen, which is where all the action occurs. You can also adjust the controls to where you move with the D-Pad and play with the A/B/X/Y buttons. You kind of have to play it for yourself first to get the hang of it, it's really a unique gaming experience that words can't describe.
To be honest, I only bought a PSP for that GTA game (which is coming out for PS2 and Xbox next month

), and I find myself only using the multimedia features (mainly internet) more than the games...
So in all, if you want more multimedia features go for the PSP. If you only want games, go for the DS.