First of all,I'll list the major differences between these two:
Stats:
Medicham-Mega: HP:60,Attack:100(200 after you count Pure Power),Defense:85,Special Attack:80,Special Defense: 85,Speed:100
Gallade-Mega:HP:68,Attack 165,Defense: 95,Special Attack: 65,Special Defense: 110,Speed:110.
As you can see above,Gallade is somewhat better than Medicham.It has more bulk and speed but is lacking in attack by a huge margin.
Moves: Medicham:
High Jump Kick: This one is probably Cham's signature move.It may be a bit risky but when it hits,it really PAYS OFF!!!
Fake Out: This one is great for disrupting the opponents strategy is used at the right time.
Gallade:
Swords Dance: Once you set this up,Gallade can be a HUGE threat.
Feint: Not sure if it is that useful but can come in handy if used at the right time.
I think Medicham wins this round.
The Decision:
Personally,I'd go with Gallade mainly because of the swords dance.Once it sets up,it covers up for the attack loss it has against Cham.But for it to be set up properly,it needs a follow me support which your team doesn't have apparently.So I would go with Cham with this moveset:
Medicham @ Medichamite
Ability: Pure Power
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Ice Punch
- Fake Out
- High Jump Kick
- Psycho Cut
However,the final decision is up to you.
Hope I helped!