What are you going to be using it for? I've got a GL3240, and a 5' RC. I, too, wasn't sure what size to get. When I mentioned I would be clearing horse trails in the woods, I was advised to get the 5-footer, as much for maneuvering ability as for not being underpowered, and it was very good advice.
At least, I've been gnawing down 2"-3" trees, and fitting between bigger ones. The RC is just about the width of the rear tires, which makes judging things in the woods easier. I haven't run a 6' RC on it to compare, but for cutting horse and/or 4 wheeler trails, the difference between a 5' and 6' hog is probably nil. Most of our trails are either one pass with a 5'er, or two passes and wide enough that in no case would an extra foot of width save that second pass...... Therefore, the 5'er is just as quick, and handles the scrub growth easier on that 25 PTO hp.
Outside of the woods, we have some treeless but weedy flat ground that I'll maintain with a 6' RFM, once I go over it with the RC to make sure there are no rocks or other foreign objects that might be too rough on the RFM.