Both are good filters.
The Wix XP is made in a wider range of applications than the Ultra. So, if there's a Wix XP available for your car but not a Fram Ultra, then it is the winner by default. For example, both Subaru-specific filters have a XP available for them.
However, the Ultra is said to be more efficient, and is usually priced lower than the XP.
If you have a Volvo, the Wix 57021XP is about $10 cheaper than the Fram XG8712, but this isn't usually the case with the XP costing less than the Ultra.
Also, the Pentastar's Fram Ultra uses a plastic mesh not the usual wire-backing. Wix sticks to the wire mesh for the Pentatsar XP.