To me, the weight difference between MB and MBA is night and day. MBA can be easily picked up just using two fingers of one hand.. while 13" MB feels like a brick in comparison.
But if you've successfully convinced yourself that 50% weight difference b/w MB and MBA is insignificant - by all means go for MB
Oh certainly the weight difference is noticeable, but as you get lighter, you start getting diminishing returns. Carrying 2 kilos compared to 1, is more acceptable when compared to carrying 3 kilos compared to 2. But everyone has difference preferences that is for sure.
Basically what I want from my notebook is performance in as light a weight packaging as possible. I sincerely hope for a kind of MBA update inline with what Scottsdale has described (cpu bump, 4gb ram, bigger ssd). If we can get that in a 1.something kg package then bring it on. If not then with the updated screen the UMB is now something to seriously consider.