Uh, Genesis Mercs doesn't have a two player mode, it's a one player only game.
That said, I've played a lot of Mercs and it's an absolutely fantastic game, one of the best run & guns of the time. Bloody Wolf, though, I'm not sure... I haven't played it nearly as much, but what I have played of it doesn't seem nearly as impressive. So probably Mercs is better.
However, Legend of the Valkyrie... that game is probably in the same genre, and it's every bit as good as Mercs.