I think Mrazek has been slightly better but jesus some of you blow everything out of proportion.
I think it has more to due with watching Howard for the whole of his now 7 years as a starter, knowing this is pretty much what you're going to get from him, and then watching Mrazek get called up last year and 1) slide into the backup role 2) Play well enough to make it a 1A/1B situation 3) Win the starting job for the playoffs and now 4) Playing slightly better than Howard and giving the team a slightly better chance at winning. Add in that this is his first full year in the league and well....(I mean Howard did the same thing to Osgood. First full year in the league was backing up Ozzy, but halfway through the season he had become the number 1. The only difference is Ozzy was clearly at the end of his career, whereas Howard is not.)
If you had two pitchers that were both pretty good (like either one would be the ace of the pitching staff if they were on different teams), you're still going to want to start the one who is slightly better (or at least playing slightly better at the time).
It doesn't matter how much better
Slightly better is still better, and the one who's better should be given preference. Mrazek is slightly better. That's all there is to it.