I've heard "tippyness" mentioned many times in relation to the 3x20 series. first day I got mine I backed over a fairly large lump of snow / ice with one real wheel and got a bit of a startle. I moved the (R3's) rear wheels to "wide" and found the fronts hit the 72" MMM on a tight turn when in wide so put them back to normal. Since then I have become used to the machine. I only have one area with a slope - never measured actual degrees, however my experience is the 3320 is less tippy on the slope than Craftsman riding tractor. I usually mow with the FEL / Bucket and BH still attatched so maybe that also helps with stability. In one place I sometimes drive straight up the hill with the Craftsman and in a couple of spots the front end feels very light. Same place with the 3320 the front does not feel light.
Not sure what you mean by scalping weight. You may get a bit more scalping with 72" than 60" just due to the physics of a longer straight line is more likely to hit high spots. The MMM on the 3x20 floats, so it doesn't really matter what the MMM weighs.
In many ways my 3320 is a giant yard toy, however, considering I started looking at JD riding mower / 2305 / 2320 / 2520, I've never regretted for a moment going with this series. At the time they didn't have drive over MMM and Cab / BH were not compatible, so only thing I'd do different if purchasing now would probably go with Cab, although it also adds some restrictions.