I'm curious, what makes Broadwell so significant?
It gives even more battery life? I personally will use my 15" rMBP mostly with the power cord in. So that wont be that useful for me. Processor powerwise I heart the differences wont be large to Haswell. Only that maybe you can have a quad core in MBA and the 13" rMBP, but this again would not affect me as a 15" rMBP user.
IGZO also would add battery life and more resolution, but I've heard also there are some problems with it, like viewing angles. Also, it is not certain if Apple will use IGZO monitors or not. For me the current rMBP resolution is high enough.
Of course the iGPU and the dGPU will be more efficient. This is something I wont argue with 😉
I doubt DDR3 and DDR4 speed differances will be very noticeable in real life usage. Also, very few people require 32GB.
Is there some other advantage that I am not aware of? (This is very possible!)
Reasons to get Haswell rMBP over Ivy Bridge rMBP:
-AC wifi
-Super fast SSD
-Second iteration of the rMBP: maybe with some of the kinks ironed out.
-Thunderbolt 2 could be useful if they ever release an external GPU 😉 (though it is not certain if this will be in the Haswell rMBP or not)