It slightly depends of preference. Medicham is garbage without it’s Mega Evolution, but with Mega Evolution it can be quite good. Then Breloom has an enormous attack stat, beating Medicham even when it is in its Mega State. So, if you want a speeder option, then go with Medicham, but if you want the better attacker, go with Breloom.
Breloom
Role: Bulky Physical Sweeper
Breloom @ Toxic Orb
Trait: Poison Heal
EVs: 252 Atk / 128 Spe / 128 HP
Jolly Nature
- Seed Bomb - STAB
- Drain Punch - Recovery and STAB
- Facade - Coverage
- Spore - Setup
Medicham-Mega
Role: Fast Physical Sweeper
Medicham @ Medichamite
Trait: Pure Power
EVs: 252 Atk / 252 Spe / 4 HP
Jolly Nature
- Zen Headbutt - STAB
- High Jump Kick - STAB
- Poison Jab/Fire/Ice/Electric Punch - Coverage
- Poison Jab/Fire/Ice/Electric Punch - Coverage
In the end, I would go with Medicham. It’s ability in Pure Power is very helpful, and sense it can outspeed Breloom, even with a little less attack, it would make it worth it.
Hope I helped!