I would have to say models on this one.
Unlike sprites, models tend to give us much, much better ideas as to how a Pokémon actually is. In former, we got to see only a glimpse, so to speak, but in latter, things are expanded more. There are many Pokémon I had previously disliked primarily due to how they had looked (one example would be Stunfisk, which had seemed rather unnatural in my eyes), but models, thankfully, helped change my views on them.
Sprites, I do like, but mostly because of nostalgic, just like almost everyone else that had posted thus far. Although I do prefer models, I would still like them to keep using sprites alongside, one way or another (perhaps in Pokémon's dex entry page?). Hope I'm making a bit of sense here, haha.