Well that is probably everyones questiion.
Why Choose one over the other? What makes one better than the other for Custom stuff?
Of course this is the Nexus forum so everyone will be leaning that way here.
I debated between them. I'll list my personal pros / cons below!
Razr
Pros: Great build quality, I like motorola
Cons: Did not feel good in my hand at all (due to width / shape I assume), Lower res screen, Have to wait for updates or Root / Rom, Can't change battery
Nexus
Pros: ICS and Quick updates, large high res screen, bigger removable battery
Cons: No sd card slot, was weary of samsung build quality, new OS / quick updates could potentially mean more chance of dealing with bugs
Either way though, I went with the Nexus... I figured despite loving Motorola's quality, I would try something new, and the pros for the Nexus were pretty important to me. A lot of people were particularly disappointed with no removable storage, but I personally never even took the SD card out of my OG droid so I think i'll be fine!