The non 'P' model AC1900 has a 1 Ghz CPU whereas the Best Buy AC1900P has a 1.4 Ghz CPU. I don't think there are any other major differences in the two models.Resurrecting the thread.
Thank you everyone, for the comments/suggestion. Since Acer was recommended often, I got T-mobile branded AC-1900 router (refurbished).
From what I read, it is rebranded AC-68U. I am not too sure if its same as AC-1900P recommended here from BestBuy. The router is in shipment. Should get it in 2 days. I plan to flash it with Asus stock or Merlins whichever works best.
Any precaution I should take before flashing? I will backup TMo firmware just in case I have to return it within the return period.