Hi RB,
I have the larger R-4's...
More flotation, higher ground clearance, less soil compaction, 4 plys...
Smaller R-4's...
More traction, 6 plys, shed mud little better
Everything holds mud except AGs though.
If you are mainly mowing, then go larger... if more loader, mud, and woods work, I'd go with the more aggressive 6 ply.