I've seen them both and totally agree. Both are two of my favorite bands also. When I saw Gojira they freaking killed it - extremely tight, with killer tone and great energy. I've seen Mastodon 3 times, and each time they've left something to be desired in the guitar tone, tightness, and vocal department. Bill actually had a really good tone during one of the shows I saw with two JCM800's hit with a dirt pedal, but Brent's blend of a Marshall, VH4, and Orange dominated the mix and was total mud city. They had ok energy, but nothing like Gojira. For one of the shows Mastodon followed Opeth, and Opeth completely outclassed them on all fronts. I really love Mastodon too, so it bums me out that they aren't better live.