Though Leafeon is speedier and has great defense Breloom can outshine it
-If you train Breloom properly, speed shouldn't be much of an issue
-Breloom has great attack
-Breloom gain great ingame moves, like drain punch, and seed bomb to counter leafeon's leaf blade
-It gets two great abilities, three if you can manage the dream world, and it can be effectively used in competitive battling
-It gets spore, which is simply amazing, a hundered accuracy sleep inducing move, also means that breloom can easily finish an opponent ingame
-Its a friggin mushroom boxer
Here's the set I'd use with Breloom: I personally recommend toxic heal
Breloom@Toxic Orb
(Optional)Adamant Nature
Ability: Toxic Heal
-Stone Edge
-Drain Punch
-Seed Bomb
-Spore
I hope you see why I like this guy
Leafeon is still great and very usable, so feel free to use him if breloom doesn't suit your fancy