I think that you need to determine what your higher level goal is for a decision like this. Personally, MonoTouch was the only path for me, as I am a .NET developer working at a .NET shop so my company would buy-in for this but wouldn't want to take a risk of doing it in Obj-C.
I know very little Obj-C, but I can say that everything offered in the BCL (Base Class Library) makes coding in MonoTouch easier. With support for LINQ, Generic lists, File IO, etc..., going .net seems like a pro (in my opinion).
In the end, you will be overriding the same methods and calling the same methods to do what you need to do things in iOS. However, MonoTouch wraps some functionality making it easier to do things.
Take into account that my point of view is highly subjective as I am a .NET developer.