Nobody has mentioned it yet, but there is no 42mm 'UPO'.
The 45mm UPO is an amazing Rep, probably one of the best reps of any watch period. It's virtually a perfect replica to the orginal.
There are 42mm PO's about, but they've all got obvious flaws. (helium valve position, size of the crown, etc). They're still very good reps (the PO is a simple enough watch to copy), but a trained eye will spot them as reps straight away. The UPO requires close inspection.
There have been rumours of a new 42mm rep that is as good as a UPO, but it's now nearly August and so far it's just that, a rumour.
So anyway, my point is, if you want the best Rep you have to go for the 45mm variant.