We have a both 6150R soon to be 6155R and a 6155M and both have loaders.  I don't know what to tell you other then I like both.  Obviously the 6R is going to be nicer to run with the suspended axle, the IVT, and a few more creature comforts.  But the 6M is not shabby either.  We bought the 6150R first to run with our Premium baler.  We bought the 6M later because we just needed a 2nd loader tractor.  The IVT is super nice but its also a peace of mind running the 6M knowing there is less to go wrong and having an old fashion gear transmission.  It brings back memories of our 7410 we used to have which was probably the best loader tractor we ever had.  Being able to shift through the gears on the loader joystick is nice too.  The 6R is going to take more book manual reading to run with all its Auto modes on the PTO and engine rpms.  The 6M you can get in and go once you figure out there are 3 different places to shift gears through the ranges.  Basically it boils down to if you want a fancy tractor in the R series or a no frills tractor in the Ms.  Both have worked great for us.