Some people prefer pure android compared to Touchwiz or Sense. In the case of the Nexus range, they get the latest updates as soon as Google releases them. But Google edition devices still have a wait.
There may be a speed bonus of a pure android device since it doesn't have a (usually) bulky skin applied on top that uses additional resources.
I have a Nexus 7, I love it. I find Samsung's Touchwiz interface to look very dated and unnecessary. (for example) That's personal preference. You may enjoy Touchwiz or HTCs Sense.
I find both TouchWiz and Sense annoying. Overall these "skins" and manufacturer bloatware on Android drive me nuts and lead to lost frames/lag/lower battery life. It's like the crap that comes on retail PC's that no one wants but the manufacturer thinks is innovative. On top of that, consider how non-sturdy and easy to break micro-USB ports are.
I had a Nexus 4 and Note 2 and while the Nexus 4 was nice, the build quality was definitely behind anything Apple. Samsung's displays are way too saturated, strain my eyes, and frankly...I don't need to be lugging another bulky 6"+ thing in my pants.