Rek'sai has a stronger engage, but Rengar with his W can become a brick in teamfights, the way Rengar's W works is that at rank 5, when he roars he gets 30 extra armor and magic resist, and then an additional 15 armor and magic resist for every enemy champion or large monster caught in the roar, so he can get about 105 armor and magic resist for 4 seconds if he roars on all 5 enemy champions, making him a tough engage champ. Although his Q is an auto-attack, flat damage, armor pen and CDR are better for you then trying to stack on-hit effects.
Rek'sai doesn't have any form of defensive buff of her own, but good damage when fed, and scales well with defensive items, not to mention her AoE knock-up, making her engage very disruptive, great for catching others out or just starting teamfights.
Warwick has good pushing potential with his E, and is very sticky, his ult is a wonderful way to peel or start fights if you're tanky enough, no defensive buffs on him either, but your sustain should make up for that, he's also a nasty duelist when ahead, and can shred most targets easily in a 1v1.
Rengar's downside, much like WW's is that he doesn't have an actual escape,(After using ult.) making him a prime target in teamfights, so consider some defensive items or a GA to stack with your W, and you should be fine. Rek'sai is handicapped in tunnel form due to the reduced vision, so try to spam out Q and wards, and play safer in general.