Comparing Mathias to Gabriel is like comparing Griffith to Guts,at least that's how I see it. If you're familiar with Berserk, there's a lot of similarities between those characters .
Mathias is described as a genius tactician in the LOI prologue, but I doubt he would have survived at that time if he wasn't a good fighter. Maybe not as good as Leon, but he could prolly kick ass. Griffith is pretty much the same.Except he's more selfish and
he's also a rapist .Gab has been manipulated all the way, just like Guts, and they're both pretty strong. Maybe he can't wield a giant sword like the Dragon Slayer, but he's a Belmont. Enough said.
My point is we can't really say who's better, since we don't know Mathias that much, and because both versions are good in their own way and don't have that much in common, besides the fact that in theory it's the same person, and they both lost their wives.
Just something I've been thinking about while reading the comments. It's also 3am in here, so I'm not thinking right