I would choose Z just because I prefer Sony concept.
Stock Android is less interesting and less pretty.
I also prefer Sony's cameras
Plus the design, Nexus always looks like a tasteless brick.
a w a r e
SE P800 P910 P990 P1 X1 X2 X10 LG E900 Xperia Arc X10 Mini Sony Xperia S, Xperia T, Xperia Z