Here’s the core issue: Xamarin’s QA screwed up with the beta channel on Windows. If you use any portable class libraries in your projects (even if not at all Xamarin related!), VS will fail to load them due to a Xamarin bug. This is pretty poor and should never have made it to beta.
That said, if you want to use Xcode 6, you need to use Xamarin.iOS 8 on your Mac. Problem is that the stable version of Xamarin for VS won’t work with that…needs the matching beta. If you try to use Xcode 6 with the stable Xamarin.iOS, it can’t debug…so bam.
I’m sure this will be cleared up very shortly, but you might want to refrain from either installing Xcode 6 on your Mac or using the Xamarin beta channels for the next several days. Version 3.6.197 on Windows is the bad one….they say it’s fixed already, so the question is how quickly can the push an update out.