only differences i've seen are price and RGB options. the Pro L's come in RGB or solid white, solid white being cheaper by a bit. the Quickfire XTi is only RGB. all of these are using brown switches.
i doubt you'd notice much difference between the two series.
i had a Quickfire XT(no "i") that was solid white backlit with brown switches and it was nice, lasted for years. compact size with no numpad. only reason i got rid of it was because a free G.Skill KM780 RGB came my way.