Gallas is defenitely the better defender, especially a really declined Marquez now. While Gallas has occasional brainfarts while being reliable quality defender rest of the season, Marquez defensively is more or less a clear liability at all times. Too slow to react (way too slow) and position himself etc. There is little to no small upside with him, error prone, injury prone, in his 30s. He has more the footballing qualities to play in midfield then defense when he is so off it defensively in last seasons.