I've made my decision and it was easier than I thought.
First, FAR more important to me is 'user experience' over spec's. Spec's are important so I can work quickly and efficiently. Otherwise, I don't care ... if I don't like the keyboard, screen or trackpad what's the point of drooling over what's inside.
To address the Sony Z first. Even though I've adjusted the brightness I find the 2010 Z screen sub par to recent years, high resolution yes, but grainy and dull and surprisingly reflective to how I recall my last Sony Z. The 13" MBP I'm typing on now is FAR easier to read text and view web pages etc.
Not only because of the screen, also the Sony trackpad, it sucks, a mouse is a solution to that, but for portable use, I'll take the Mac trackpad. Finally the Z keyboard is nice, better that most pc notebooks for sure, but not quite as nice as the MBP. OS wise I was so anti mac at one point and Windows all the way, I'm shocked myself that I like OSX so much more than Win7. I won't go on about OS's, it's personal preference.
15" MBP
It's a heavy, bulky trade off to get the spec's of the Sony Z for sure. If I had to buy a high spec'd machine and choose between the Sony Z and 15" MBP I'd probably take the 15" MBP and bitch about the weight and big footprint. The screen, keyboard, trackpad - user experience is better for me on the 15" MBP. I can't choose the Z even though it's small and light if I don't like using it.
MacBook Air
If it had the spec's of a Z there would be no 13" MacBook Pro. I love my Air but short battery life, lack of ports and ram make it a machine I'm passing on to one of my staff.
The Winner - 13" MBP
I realize this is a Sony Z / 15" MBP thread ... I came into this thread with both those machines, but I'm leaving with the 13" MBP and thought it may be of interest to others who can't decide what to do.
First, I considered spec's - what notebooks will do the tasks I need to do 'lighting fast'. The Z and 15" MBP meet the criteria of course, for me the 13" MBP, with max'd out spec's, including 256ssd and 8gb ram does as well. Gamers and video editors may need more power.
Next I wanted the thinnest, lightest option available and from a user experience point of view I had to ENJOY using it in all or as many situations as possible. The 13" won over the 15" MBP because I find the 15" not as good to "take everywhere" or use on my lap on the couch, airplane tray or coffee shop. The 13" is small and slick to pull out of a bag and start working on. With the 15" I found myself thinking twice if I should bring it, or I'd consider bringing the Air - lacking battery life, or my iPad - lacking a lot. When I need more than a 13" screen I have my 24" ACD, best of both worlds I think.
That's how I made up my mind. The 13" will easily replace my 15" MBP, my Air and iPad too for that matter. It's small enough to take and use and when you open it up you know it's got the power to do the work, a great screen and keyboard, and the battery life too. win - win - win. I'm happy to be going back to a 13" MBP and very glad to see the Sony Z go back.