From my understanding after having read through other groupbuys & posts on this forum:
the U2+ is pre overclocked at 2Ghs by default, hardware is identical to the U2. I assume older batches (such as the ones for this preorder) are the original U2 and newer orders come "updated" as U2+.
It's a bit confusing when Bitmain themselves label them as U2+ on their website
Only thing changed is the heatsink, they finally ran out of the "better" original asicminer .333 blockerupter H/S that they made billions of.
Antminer just reprinted them.
The original design was better in one respect, smaller more room in hubs to stack em, they did not overheat.
I believe the U2 is simply WoW factor but foolish.