I've never played either, but I'm getting more and more interested in the Sly series. It seems that from this generation Sly gets more love and has more fans than Jak & Daxter (especially judging from the ps3 HD collection sales of Sly compared to J&D); but when all these games originally released on the ps2, the Jak and Daxter games sold over 2 times more than the Sly games.
I only have time currently to check out one of these series. The Sly series seems more interesting to me than J&D, but it also doesn't make sense why J&D sold so much more if they weren't really the better games. Opinions please:)