I truly despise GW's style. It's solely personal opinion, obviously, but their style really sets my teeth on edge. The one exception is their LotR line. Those are some beautiful minis, but I wouldn't say they're any better quality than Reaper. Reaper, like any other company makes some figures I don't like. Some of their minis are somewhat over the top (giant anime swords, etc.), but they produce far more minis in a style that I do like.
However, as others have stated, the biggest difference to me is the quality of the company and the people that comprise it. GW's customer service may be good, I've never had occasion to find out, but I can guarantee it isn't better than Reaper's customer service. But GW's other practices turn me off even more than their figure style. 25 dollars for a resin man-sized figure simply because he costs more points in their game? Gimme a break! On the other hand, when the price of tin started climbing, what did Reaper do? They gave their customers the option of buying many of the figures in a lower-tin-content version to help bring prices down. Now that is a win!