Pro IIs in the front, good idea: fit and forget, replace the bearings once every 2 years, and they are only slightly more expensive than XTs (and XT doesn't come in 20mm).
XT on the rear is £80 less (about 1/2 price!), and only really needs the bearings replaced annually unless you do something stupid, whereas the Pro II needs new bearings every 2 years.
Unless you are using one set of wheels for different bikes with different axle designs then the Pro II isn't really worth it unless you are a colour maniac and don't want a black or silver hub)
The noise on them is really nice though, but I'm getting wheels for what my buddies call "a bag of spanners" (Scott Octane) so it'll be noisy whatever, therefore XT for the rear, ProII for the front.
All prices are based on CRC.