It probably depends upon Where you are and what if any 240 Volt receptacles you presently have.
I believe that the 14-50 is the newer style. It can handle 50 amps. The other, I believe only goes to
30 amps. The "R" is for receptacle "P" is for plug. If you have something there already and don't
wish to upgrade, tell them to send you what matches what you have. I had the old style but updated to
the 14-50 just because. You won't be able to pull 50 amps ( 40 actually, 80% of 50 ) unless you have
breakers and wiring to match.