Since you have a "speedier" playstyle, I would recommend Breloom. Not only does Breloom have a higher speed stat than Walrein, it has a great movepool in Leech Seed, Mach Punch and Stun Spore. Breloom's Grass and Fighting STAB is good against:
- Wattson's Magneton (Fighting)
- All of Norman's Pokémon (Fighting)
- Tate & Liza's Claydol, Lunatone and Solrock (Grass)
- All Juan's Pokemon excluding Kingdra (Grass)
- Most of Wallace's Pokémon (Grass)
- All of Sidney's Pokémon (Fighting)
I don't see why you wouldn't use both, though.. They don't have any type overlaps, and Ice is good coverage, considering there's only the Glalie line and the Walrein line that are Ice types. Also, why are you using Breloom or Walrein? There are much better Grass types like Ludicolo, and much better Water types too.