kfc wroteLifeEngineer wroteWell bro I can reply about the nexus 4! I did not try or even see any xperia Z/ZL till now!
Nexus 4 is fast, snappy, great screen (i like far more than Samsung Amoled which is over-saturated) , responsive.
I also like the smoothness of the UI. It practically rarely lags especially in UI. Everyone who tells you that it doesn't lag don't believe him. Every android phone lags unfortunately! However the nexus 4 is THE LEAST lagging phone I have tried. Even S4 lags!
About battery life: it's not awful but not great as S3 or S4 or other new smartphones!
Now it all goes down to you and your usage:
Do you care about regular android updates and latest version?! Do you care about custom roms and kernels?! Do you like customizing and tweaking?! Do you NOT care about those gimmicky features and 4G!?! Can you live with relatively slightly lower battery life?!
If the answers are YES like me then Nexus 4 is for you! It's a wonderful device!
If not, I guess it's not!
Hope I helped.
Thanks for the feedback! What about its resistance to falls and shocks? I read somewhere that it is quite fragile. Is that true?
Also does it last 15 hours of regular use without need to be recharged?
well I hope I never know about falls! :-P I've never dropped it till now!
I know the back is glass so I take extra care for it! After around 3 months of ownership, I dropped my previous S3 like about 5 times! :)
However yeah it might be a little fragile so get a nice cover!
If you are a heavy user and you want, I can give you a special cover I got from USA but used it for like a week only. It was too bulky for me and ruined the shape of the phone.
It is the Mybat TUFF Case seen here:
http://nexus4accessories.com/the-most-rugged-nexus-4-cases/