it's always been my experience that when you buy smaller it doesn't take long to out grow the machine and wish you had gotten the larger model. I've seen a lot of guys buy smaller then start looking into mods to make the machine better stronger faster and end up spending a lot more in the long run .... I know it doesn't answer your specific question but I hope it helps you with making yer decision when buying
Don't know if you finally got the 550. I have had mine for a year now and it has everything I need. I weigh around 230 and my dad weighs around 270. No problems getting up hills and light mud. I also have a 100 gallon tank for the bed that we use for collecting sap when making maple syrup. The suspension doesn't like the full tank, but the engine doesn't seem to mind at all.
My 550 S4 does anything I ask to. Deep sand no problem. Need pull out some Sago please easy. It's a tuff little rig. Had some transmission problems but JD fixed those. It has plenty of power and 28 mph is good enough.