He had a shaken start of the season, but he's playing well now. It's hard for me to say who's better, because Bravo minutes are consistent, not cup minutes.
Both are on a very similar level, to me.
In terms of ability, I'd say they are close. Stegen has more talent and the potential to be the best keeper in the world one day. For now, Bravo's experience and his personality, as well as being from a latin country, with a similar culture to Spain, is what makes him the better option, at least in La Liga, a competition he knows inside out (the language, the stadiums, the atmosphere, players etc. etc.).
I'd also agree he improved after what was a pretty rough time for him at the start of the season, but it's also importat to say that, when he had to step up and show he can be our no1 keeper in all competitions, he failed, and fans missed Bravo in the goal. He's not ready yet imho, but he will be. It's up to him, because I don't think the club is stupid enough to sell him.
He won't be sold unless the club is utterly stupid.
Or he wants to leave. When you don't play in the most important competition of the season (La Liga is about 70% of the season, all sure games), you could think about leaving. It's up to Lucho to find a way to keep both as happy as possible.