Thank you much for the message but now I am none the wiser OAO there is an R in both types.Well, the short and less than complete answer is that the R stands for Resistor. Thus these are two similar plugs except that one has internal resistance and the other does not.
I had the same issue with my R171 with an M272 engine. The Owner's manual spec'd the non-resistor plug, but the only place I could find them was the dealer. I read on here where several people had used resistor plugs in the M272 and said that they had no problems, but for where I was at I spent the extra $50 and put the non-resistor plugs in my car. FWIW when I took the plugs out which were possibly OEM (43K miles) they were non-resistor.
Beyond that you will need to let someone more knowlegeable weigh in.