i have seen the z5 before, i recommended it to my student and he brought it to class.
generally, ppl who are alright with 15" wont find it big but most people (those in my class at least) generally use smaller laptops around 13-14" being on the go (most using Macs).
the Q3 seems to hit a really sweet spot here.

ps: personally, i did not like the Z5 design especially the internal color scheme