We have a 6155M and 6155R.  Big differences between the two is the R has the IVT transmission, front suspension, bigger cab, fancier cab, cab suspension.  Because of the nature of the IVT, I'm guessing there are going to be more computer/controllers on the R vs the the M.  There are probably some other differences I'm not aware off for example I'm assuming the hydraulics on the R have more capacity but I'm not sure.