It's because the Sony Z uses the new HM55 that is slowly (slowly) making its way around. Most of the Arrandale laptops on the market so far use the older PM55 chipset (I think this includes the Envy, although I thought it had an HM55 for a while lol).
I can't give a direct comparison since I own an F series, but I like the Z more and more everyday, it's more powerful than a 13" MBP but it weighs like a MBA. Gotta love that, lol.
Sony's support is actually pretty good (I was surprised), but if you call after hours (since it's 24/7) weird things happen. Oh, and the last Z was made in Japan (or in California if you custom ordered one), so I am assuming the current Z is also being produced that way, if it matters much to you.
Either way, I've heard great things so far, but you may find yourself upset when you realize it's not a Mac lol 🙂
The last time I owned a Vaio, it was the first desktop tower they made, I still have most of that machine laying around and it still works great, but there was one flaw - the tower's side door always kept falling off lol.
I beat the hell out of that machine though (not PHYSICALLY but still lol 😀). Many, many experiments and the dang thing still works so.. yea. XD