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