I remember back when I was trying to decide on a Nexus 4 or Nexus 10. Ended up choosing the Nexus 10, and it was pretty awesome

except for stuff that required mobility (jogging, Ingress, etc.). No mobile data is also a slight pain in certain places.
Just got a Nexus 4. It'll be real nice for mobility, but gaming, reading, and media are
really lackluster, in-comparison to my Nexus 10 (I'm used to 7-10" tablets though to be fair). Mobile data is possible though, and as is tethering to my Nexus 10.
So basically, both devices complement each other in my situation. I didn't really require a phone when I made my first purchase though (had some cheap free minute phone).
If you require a phone and know you'll be in areas without internet access, I'd say go for a phone. If you require a phone and know you'll always (or most of the time) have access to internet, the Nexus 10 + Google Voice would work nicely. If you don't require a phone and want a large tablet with excellent screen quality, the Nexus 10 is awesome. And if you want a balance of mobility, large screen, and phone, the 3G Nexus 7 is pretty much perfect.