Nexus devices usually run a pure version of Android, a situation that has its pros and cons. "Off-the-shelf" Android is still a *really* good system, but some manufacturers (I currently have a Samsung) have added a few attractive bells 'n' whistles to their version of Android. This is good if you like the additions, less so if you don't. But it also means that you might have to wait longer for the release of a modified version of Android. The Nexus update would come from Google, while I'd expect the Motorola update would come from that company.

Both my wife and son have benefitted from having a warrenty--I, otoh, have never had to return or exchange a phone, but I can certainly see the value.