Which do you like more?
They both have a single target bite.
Both have AS modifiers
Both have MS modifiers
Both have AoE slows.
Both have self healing passives.
Both can carry equally hard if they get ahead, but both can be kited. Biggest difference is in their ults.
I seem to do better with trundle. I think mainly because of his E because ranged and able to zone so well with it, makes for singling out 1 or 2 enemies and killing them quickly. Also his W is just amazing. Unlike voli, he gets his full AS boost immediately and gets his MS boost at the same time.
I like the fact that voli scales with tanky items much better than trundle. Trundle seems to need at least triforce and/or botrk. Voli can forgo any damage items and still be a massive threat.
But I think voli is more prone to kiting. Once he hits his Q, if he gets CC'd or slowed then his gap closer is gone until off cooldown. Trundle's doesn't go away that easily and with CDR he can have it up all the time. He can also still AoE slow from ranged, while voli can't if he gets kited.