bassnyc1,
Did you order either? According to their website and latest specs (these often change every few years somehow, yet they probably remain the same product), the Relentless Middle that Mr. Billy first co-designed with Mr. Larry has a bit more low mids-the Relentless P consequently has slightly less low mids, and more treble. The original 1977 Model P has "better" treble than both, and less low mids than either Relentless version. This could all be up to interpretation, as once they measured in only bass/mids/treble approximations.
The low mids are impressive on the Relentless P model, yet not muddy. Never had a Relentless Middle equipped bass, but the specs seem too close to expect a stark difference-main benefit would be the lack of touchable pole pieces (though the latter is also a feature more than a defect, since these are adjustable, as with the Model P). I am glad they did not retire the old and super well-known Model P, though.
I would happily use all three versions of this model, though the radiused aspect makes both Relentless models *perfect* for my playing. Wish more pickups could be used like finger ramps, at least as well as the Relentless style pickups do.
If you already did make a choice, I doubt you made a mistake. Just two versions of excellent after all.