In competitive battles, both Slaking and Vigoroth have their advantages.
Slaking is obviously one of the most powerful Pokemon, having some of the highest stats around. It's Truant ability does limit it though, unless you are going for a Skill Swap 2v2 tactic.
Vigoroth is useful in battles for it's ability - Vital Spirit. Virtually shuts down some Pokemon like Butterfree who are almost solely dependant on sleep attacks to get anywhere. The stats aren't too bad either, coupled with a Normal type move for STAB can be very reasonable in battles. Just keep him out of 2v2.
Basically, it's your choice.