If you already know C# and have $400 to drop on Mono Touch, then that might be a good way to go. If you don't know C#, then you might as well learn Obj-C. At least that is my opinion.
In my experience, C# was easier than Obj-C - not $400 worth of easier though. But each person experiences different learning curves.Don't know C# at all, neither do I have $400 to drop into it.
I am finding Object-C quite tough though.
http://ios.xamarin.com/documentation/limitationsAre there any limitations to using C#