Options: Halo 4 by a long shot.
Looks/style: tricky one, Halo 4 is like 95% hideous and 5% really nice, while Halo 3 is just 100% average, then again since H3 doesn't have many options the 5% good looking ones over weigh the 100% average ones to me. Also in Halo 3 we barely see any secondary colour which is sad.
I would say that the 5% of nice looking armour pieces in H4 are more appealing to me than Halo 3s choices but the other 95% are sooooo much worse than Halo 3.
Halo 4 is just better if you ask me, but I completely get that Halo 3 "generally" looks better and more like Halo should.