neither -_-
both have some major balance issues that need to be worked out. Until then they are on my do not touch list. I used to enjoy Renekton too, but that doesnt excuse the fact that he is an unhealthy presence in the game right now and playing champions that tip the balance scales is only perpetuating the problem. At least that is how I feel.
Nasus really needs only one change and that is to his passive. I feel that Nasus should trade passives with Fiora. Nasus has the beef and bulk to actually see a benefit to an increase in health regen while Fiora would work far better with some bonus life steal at the start. Siphoning strike however would need to have a life steal effect on it though equal to his current passive at least. Siphon strike not leeching life when used would destroy a part of Nasus identity.
If I were forced to pick though it would be Renekton