From what I've read, the UMC is a bit hotter than WWB.
As far as their quality, WWB never gave me any problems... I just think it shot way dirtier than necessary. Does that even matter? I don't know, but I prefer clean burning ammo. I've never shot the UMC, because it was always higher priced. I've Actually been sticking solely to Speer Lawman. It's the cleanest burning stuff I thought, with blazer brass being right there with it in cleanliness. But now I'm getting off subject, but I always have to say this, I prefer Speer Lawman over everything because of its low price and it's loaded (ballistics-wise) just like Speer Gold Dot.
If I was in Wal-Mart and saw UMC @ 20 cents a round for .40S&W, I'd buy it without a second thought.