The max card I've gotten with threes is 384, while the max card I've gotten on 2048 is 8,192. With Threes I don't really care about not getting any better, while with 2048 I feel close to getting 16,384 and still think it's interesting to do so. Threes just doesn't have the staying power, the mechanics are simply more frustrating.
I've seen your argument before -- the big post the Threes guys did had it also, that they felt their game was clearly better. I can understand why they felt that way, but I don't get it. And given the taking over the world popularity of 2048, I'm not sure that the evidence is there to support that position.