I guess it's probably because I've been playing the games since the beginning, but I prefer sprites. It's been nice to watch them evolve across five generations, slowly getting more lively and detailed, and I suppose you could argue that the models they use in the sixth generation are the next logical step, but they just don't feel right to me...that might change with time, but right now I feel like they're a step back, rather than a step forward.
I mean, attack animations aren't hugely improved - they still writhe around in mid-air for physical attacks, etc - and the in-battle movements aren't a huge leap up from the fifth gen. The coloring for the models is a little off as well, especially for the shiny sprites. They don't LOOK shiny, they look washed-out. Sprites were vibrant and colourful. Models look quite drab by comparison, and when Pokemon is a bright, colourful game, I'm not going to accept the "more realistic" argument to justify this. They're just not as exciting.
As I say, my view might change with a couple more generations as they improve; maybe I just need to get used to them as well. But for the moment, I much prefer the sprites.