OK, well here's the long answer. All IMHO of course. First of all, what makes a plug a plug is the lower volume fit and the thicker shell as Sierra Jim said. So that means you can't really compare with as you said "fit being equal". It seems like what you really want to compare is stiffness, and that you can compare. To take your example with Nordicas, you could compare a Dobie Pro 130 to a Hot Rod Nitrous which has a flex of 100 or so. One thing that I think will really matter in crud is that you are well-balanced in the boot. Your neutral should be neutral. You shouldn't have to push into the tongue to be in balance. You shouldn't be "hanging on the tongue" as SJ said his friend was. Was that technique, or a poorly balanced boot? I feel that in my 150s I am balanced right in neutral without having to try. That means that I can make minute adjustments off of the stiff cuff to maintain my balance when skiing in chop. If I needed to push into the boot to be in balance, maybe I'd want a softer boot to allow that, but I think it would still be harder to remain in balance.
Hope that makes sense.
I skied with your MRG ski partner (ilikeski, right?) a few days after you did in a nice dump at Stowe he could probably tell you how pace and terrain compared. I'm not sure quite how big you are, but I do think the XT 24 is a very stiff boot, and if it's anything like the Diablo, it has a lot of forward lean. I think I'd be a disaster in those boots. FWIW, I am 200 lbs and have had the 150s softened a bit so I don't lose any dental work when things are frozen real hard.