What's better?

 

Full Melee Void with Whip and Defender or Bandos armor with Warrior Helm, Whip and Defender?

Depends where you are training, simple as that.

 

(Nezzy helm would be better than warrior also)

It is just for fun fighting really. Such as Castle Wars, Soul Wars, POH Dungeons. Fight Pits etc.

 

(I prefer maximum Slash bonus though so I choose Warrior :P)

Bandos :?

 

It's much easier to stay alive, and will in the end help your team out more. If you are constantly slapping those band-aids and needing to restock/suicide you are spending less time fighting. Might as well use bandos because it's only slightly less offensive but has tons more defense.

If your opponent fights backs, bandos. Otherwise void is better.