Well, lets see.
Mega Gallade has higher attack
Mega Lucario has higher defense
M-Lucario has higher special attack, as well.
M- Gallade has higher special defense
M- Lucario has higher speed
Now for weaknesses and resistances
Mega Gallade has Flying and Ghost as its weakness while Lucario has fighting,, fire, and ground as its weakness.
Gallade is resistant to Fighting, and rock.( This means Gallade is resistant to Lucario"s fighting type moves)
Lucario is resistant to Bug, Rock, Dark, Dragon, Grass, Ice, Normal, Steel.
Overall, it seems as if Lucario is best. It has higher stats than Gallade, and can cover weaknesses easily. Even though it is weak to Gallade, it has many resistances.
Hope I helped!