Hawkfeather, I gotta say I quit reading Reds drivel a while back so I'll just throw in my opinion and experience on your real question.
The MS260 is a great saw, in fact my 260 is the one saw that goes with me every time I cut and I burn more fuel in it than any other saw I own. I think a 20" bar on a 260 is asking a bit much, it will pull it just fine but you will be working the saw harder than I think you would want to. The 260 is best with a 16" and an 18" isn't to hard on it, but a 20" is starting to push it a little.
The 361 is the better choice here and will pull that 20" bar very nicely. Yes the 361 is a little heavier but you will actually work a lot less with it due to the fact that it will get the work done faster, and you're not holding the weight anyway when it's in the wood.
My opinion/recommendation get the MS361 with the 20" Rollomatic E bar and the RSC chain, and stick with the factory 7-pin sprocket that comes on it. Get your dealer to throw in a spare chain and some bar oil and mix oil. He should give you a discount on the case, some chaps and a helmet with muff's and a face screen..... and you want/need all three. If you have to go without one of them make it the case. You should also pick up a "chain maintenance kit" from your dealer while you at it (part# 5605 007 1029).
I know some have recommended the ES bar and it's a great bar, I have eight or nine of them myself, but the E bar is a tad lighter and you said weight is an issue. The E bar will probably last you a lifetime anyway, but if you do manage to wear it out it will be cheap enough to replace in say 10 years or so.
Just my
Good luck with your purchase, you can't go wrong either way.