From the rumors I've heard, Aprilia will soon be replacing the 60 degree with a 90.
I've owned an RC51, and I always lusted after the Mille's. Truth is, the RC51 outperformed the Mille at the time, and also had a much nicer sounding motor... due mainly to the 90 degree set up, and chain driven cams. Best sounding twin, IMO.
The new RSV 1000's though are awesome! Fit and finish is second to none, and they're not nearly as common as a japanese sportbike.
Just make sure you put those aftermarkey Akrapovic made Aprilia pipes on it if you get it. They're fantastic.