It is clear from results between the two leagues that the Pro League is as good if not better than the MLS.
That's quite a reach... hard to make that determination based on Joe Public's defeat of a tired and depleted NE squad, and Jabloteh's rout of NY... the weakest team in MLS this season. If there are other victories that you factored into the equation feel free to mention them, these are the only two which presently comes to mind. Other than that the PFL is good to surprise the occasional MLS team, but not sure I'd put them on par just yet, let alone to say that the PFL is ahead.
I think is safe to say that the MLS is better than the PFL..... but only slightly, I doubt a PFL side would perform well consistently if put to play in the MLS. IMO the good results we get against their sides is mainly because they view us as weak opposition (just read the big soccer forums fuh evidence) as such they class their league games as higher priority (especially) when playing us and the MLS's scheduling clearly gives short shrift to the Concacaf club competitions. The MLS thinks its on par with the Mexican league, but they are wrong and clearly don't have the depth that Mexican sides have. I assume the longer they collect embarrassing beatings from our sides things may change.
Longterm they will always have the ability to improve on anything we can achieve due to the nature of America (funds, population, TV etc, its all bigger and better). Unless they horrendously mismanage the MLS's finances they will always have more financial clout than the PFL so will be able to attract better players, coaches and facilities. However, as long as the PFL concentrate on development and local coaches on producing sides that play well as a unit, our sides will always have the chance of winning the one-off games