After about 18mos living with my Rocky Mountain 3 leaf rear parabolics, I thought, for posterity, I'd update this post with my impressions .
I went against a few recommendations here (most seemed to be vaguely pro 3-leaf) and more importantly against the recommendation of (the incredibly helpful) Jeremy at RM, and got the 3-leafs. I have a pickup cab 88" SIII - and it's my daily driver and regular road-tripper. My rationale for going 3-leaf, was based on towing a small boat regularly in-season, and future-proofing (future cage/rack-storage/full length top, etc). While towing the boat (~100lbs T-weight), or loaded up in all 7 seats (or as high a pile of firewood as I can jam in the back, etc.), it handles like a dream. Smooth, stable, virtually bounceless (and quite a bit quieter).
You guys hear a "but" in there somewhere?
But, Except for the boat, those fully-loaded events are fairly rarities. And the boat tows probably average twice a week, for 12 or so weeks usually about 2 miles (though a few 20 mile round trips). I ride topless the entire time it's above about 45 (that t-shirt weighs more than you'd think - I kid). But the top isn't heavy at all. I can lift on and off solo, pretty easily. So probably not much difference there.
The downside, is that completely unladen, I have what feels like about the same ride as my original springs (except they were very fatigued - the Rover sat embarrassingly low in the rear - and would bottom out with two guys in back). But still, the ride was and is ROUGH! Especially in winter when the roads heave due to frost, and there are snow and ice patches to bounce off of.
I guess I still hold out hope for the futureproofing aspect of my plan to have been a wise choice. Though during particularly rough drives, I often daydream about offering them up for an even trade for 2-leafs or cash close enough for me to get them. I actively avoid the worst roads in town, if I can. But a bit of a rough ride isn't the worst thing, most of the time.
So my experience is mixed. I'd prefer to be able to do an A-B test on both setups before I changed anything. I have no other way to asses the unladen ride improvement vs. more-deflection-under-load compromises, except trying them. But my guess is that I'd probably be just slightly happier with the 2 leafs - at least under the current setup and use of the Rover.