I have been a Z Ultra owner for 5 weeks and i also own an Xperia Z and a Nexus 7 2012 edition.
i would buy the Z Ultra, hands down no question. Why?
First of all, my Xperia Z and its 5 inch screen feels too small now, like tiny!! Crazy right, its 5 inches! haha.
On top of this, the Z Ultra is super fast and lag free, it makes my Nexus 7 feel like a huge step back when i use it.
Basically it combines all the best things from a tablet and a phone and puts them in to one amazing package. So yeah, buy the Ultra and do not look back, you will not regret it
1 last thing, it would appear from what i have seen that the Z1 has a worse screen than the Z Ultra. Looks to wash out more easily similar to the original Xperia Z.