Nothing controversial about this topic... Neither of them are particularly good films, but the grittiness of The Wrestler gives it a shock factor that Balboa lacks. Having said that, it's still a flimsy, thin film.
Balboa on the other hand feels like a high-standard TV movie rather than a cinematic feature. The pathos and weariness it presents delivers a new shade to Stallone's characterisation and a valid new chapter - in this instance, familiarity breeds respect. It's probably the best Rocky since II, and a better film than Aronofsky's. For me, the deciding factor is which one I could watch again; while I quite liked The Wrestler, there'd be nothing to gain from a second viewing, whereas I could happily waste a rainy Sunday afternoon on Balboa.