According to B.J. Rains, Cardinals pitching coach Derek Lilliquist indicated that Thursday will be the "final audition" for Shelby Miller and Joe Kelly for the 5th starter spot. Kelly will get the start while Miller will work in relief. Not sure if that is any indication as to who the leader is, but we should know more after Thursday's game.
Personally, I'm pulling for Miller. I think he has more upside and he certainly showed in the second half of last season, both in the minors and at the major league level, that he is ready.
However, I get the feeling that the Cardinals will end up going with Kelly, for the following reasons:
First, Kelly has already proven himself as a capable major league starter, having made 16 starts in 2012 and posting a respectable 3.74 ERA in those starts.
Second, the Cardinals can delay Miller's service clock and thereby have him under team control longer.
Finally, waiting until June or so to call up Miller would allow the Cardinals to avoid the dreaded "Super Two" status for Miller.
Of course, that could all change on Thursday if Miller pitches lights out and Kelly struggles. But there is a lot more to this decision process than just who pitches better this spring.