You can't really compare them. MonoCross is a crossplat MVC framework that you use on top of Xamarin's Monotouch/droid/etc. to increase your shared code. Xamarin allows you to write about the same code for multiple platforms, but it doesn't make it easier to share the same exact code. E.g http://components.xamarin.com/view/xamarin.mobile

The code looks similar across platforms, but it's not exactly, the same - you can't make one change and have it carry across to all platforms. Monocross brings you closer to a write-once for all platforms experience.

A better comparison is Monocross v.s. MvvmCross