I voted DS-Lite, 'cause the DS has a much more compelling, much more interesting library than the PSP.
I mean, you named 2 games for the DS, and 4 for the PSP, but really I'd say there's better than a dozen particularly fun ds games that I know of, and I don't even HAVE one

.
Not to mention backwards-compatibility with the gba and its insane library of games.
Everyone I know who bought a PSP enjoyed it for about two months ... it's a nice little system with some really good specs and nice graphics and all, but ... there's just not a lot of interesting games for it.
(this may or may not hold true for you, depending on your definition of "interesting" games...)