Haven't seen much of Lillard but a Curry-Lillard swap is quite tempting. He'll be the better defender, probably gets to the rim better, probably just as good of a floor general and may end up better than Steph.
That said, Steph is just a very different player from everyone else in the league. Hes had historically elite shooting percentages to this point in his career. Hes obviously more perimeter oriented, which has bad connotations, but hes so good at it that its a positive. Hes the older of the two, yet just beginning to be the primary ball handler on the team, and hes shown growth there. Hes a mega elite shooter, and you wonder just how much a guy like David Lee or Carl Landry benefit from the extra floor spacing he provides. Hes been clutch this year and when he plays with his douchey swag hes a cold blooded mf'er. All that said, hes got some big time flaws as far as inability to get to the rim (and by extension inability to get FTs), garbage on defense, not a "true" PG, still makes a lot of lazy TOs, and of course his ankle history.
Lillard is the more traditional player, hes younger, more athletic, and has no concerning injury history. How good will he be? Hard to say. Steph's a guy you'd ideally want spacing the floor for an elite player and giving you a great second ball handler, but Lillard can be a ball dominant star himself.