MvvmLight does not have support for iOS yet. Support for Android was added in version 4.0 in May 2014. More info here : http://blog.galasoft.ch/posts/2014/05/mvvm-light-v4-4-with-xamarin-android-support/
MvvmCross does not support Universal Apps (yet). More info here :https://github.com/MvvmCross/MvvmCross/issues/656 A ray of hope here : http://www.microsoft.com/en-gb/developers/articles/week05jun14/mvvmcross-support-for-universal-apps
Looks like you have two options -
MvvmCross allows you to share code with Windows Phone and Windows 8 also. Just not via Universal App project. If the objective is to share code - you can go with MvvmCross. You wil get your WinPhone and WinStore apps along with Android and iOS also.
Start with Mvvmlight on Universal App and hope for Xamarin.iOS support soon. http://blog.galasoft.ch/posts/2014/04/building-a-universal-application-for-windows-phone-8-1-and-windows-8-1-with-mvvm-light/