You can't compare them both, even though they are both Belly Drummers
Azumarril is a priority Belly Sweeper, while Slurpuff is an Unburden Belly Sweeper.
EVs - 196/252/0/0/0/56
Ability - Huge Power
EVs - 4/252/0/0/0/252
Ability - Unburden
But if you want a verdict - I would choose Azumarril easily. Why? Because of Slurpuff's extremely small Physical movepool, it hardly has any good coverage - it only learns Play Rough, Return, Tackle and Giga Impact, of which only 2 are useful. But Azumarril has access to Aqua Jet, Waterfall, Ice Punch, Superpower, Brick Break etc., of which most are extremely powerful and are usable.
So choose Azumarril!