mestre nova is the best NMR processing software I've used (though I have used neither of the ones you mentioned) and it is available for Mac OSX.
A friend of mine told me that the Mac version of mestrenova has a persistent problem: it's very slow in drawing. He has compared the Windows version and the Mac version and the difference is marked. In the case of 2D NMR the program becomes almost unusable. This is a long standing and well-known problem that they haven't been able to solve. Probably that's the reason why the same maker also sells iNMR. Apart from this, if somebody prefers the Mac over the PC, than he'll also prefer the interface of iNMR over that of mestrenova.
The community of iNMR users is growing. Se for example:
http://www.inmr.net/#BUZZThere used to be the Google maps with all the user, but they have been removed, don't know why.