I have several mobile websites and some are .com and some .mobi. I don't see a difference in rankings, but I wouldn't recommend a .mobi if you have plans to use it for a regular website as well.
I wouldn't be suprised if Google on a given moment decides to adjust their algorithm to check several aspects of a mobile domain, like mobile friendlyness and mark up (which they should have done a long time ago imho).
"Depend on the rabbit's foot if you like, but remember it didn't work for the rabbit." R.E. Shay