So far Steyn's obviously accomplished far more. And if Sharma wasn't an Indian seamer, with all the potential pitfalls that involves, I'd back him over Steyn every second of every day, for one crucial reason - the height difference.
Both very young; both have problems with overstepping that seem to be getting better; both more accurate now than they were not so long ago; both get great movement away from the right-hander, though Steyn is the far more skillful swing bowler; both around the same sort of speed out of the hand; and both something of great-white-hope types.
But Sharma has the height that can make such a massive difference, Steyn's shortness will always be a handicap to him. Sharma has such a greater margin-for-error in length, and will be able to get the edges he gets to carry more often with his frame than Steyn.
But I honestly wouldn't want to predict great things for an Indian seamer. We've seen countless ones who appear to have some potential, Munaf Patel the most recent case (right now it might be fair to call Ishant the new Munaf, and Munaf made his debut 2 years ago FFS) but there are always a great many things that can go wrong. So right now, given Steyn's been around for longer and hence done more, I'd go for him.
No rule is without exception, though, and if we're lucky Ishant might just end-up India's greatest-ever seam-bowler.
Something I don't hesistate to say is beyond Steyn for South Africa, though that's not entirely fair as clearly Allan Donald >>>>>>>>>>>>>>>> Kapil Dev.
EDIT:
YOU GOT A SMILEY IN THE POLL OPTIONS???????????????? How... I never knew this was possible.