I swore I would always ride an inline four until the day I died. But I said what the hell and two months ago I bought a Mille, my first twin. I have ridden an '01 GSXR 750 and an older generation ZX-9R through Deals Gap. The GSXR is an incredible machine out of the box...no doubt. My problem was that I had an R1 at the time and we all switched bikes. If you like floggin the throttle to make power you will like the GSXR. You have to ride it like a 600 (almost). I am used to powering out of turns at about 6-7k rpm on an R1 - if you tried that on a Gixxer 750, you would be lapped by the old guy on an amigo!!!
The ZX-9 (granted not the latest model) was an excellent powertrain, but it was a little heavy and not as flickable as the other bikes. But probably the best all around bike in the batch when you take 2-up into consideration.
Have not ridden an RC-51.
But I do own a couple of Milles and I have to say that it is the easiest bike to ride fast through the turns - tight ot fast sweepers - that I have ever owned or ridden. From actual engine braking aproaching the turns, to the confident powering out grunt exitting the turns, the confidence is high all the time.
I am riding faster on my Mille in two months than I ever did on my R1. I never fot the R1 to handle well and I had a ton of upgrades to the chassis.
The bottom line is that for me, I may never sway from a twin...of course I said that about inline 4's! The only complaint I heard about the RC51 is that it is underpowered. This is from a Duc 916 owner. Truly a beautiful bike though.
As far as riding 2-up, they all suck! Its just different levels of 'suckdom'.
IMHO, the gixxer (if you like to ride 600-style) or RC51 (if you want to grunt out of turns) if you're going to canyon carve primarily. The ZX-9 if you want comfort and a more all around bike for commuting, twisties, and 2-up.
I hope this helps