I have had a Rolleiflex and currently use a GF670. For me, the rangefinder is superior because it is much quicker and easier to focus. In the 100 rolls I've shot with the GF670 only a handful have been out of focus, and that's been due to error on my part. With Rolleiflex and Hasselblad, I found focusing far more difficult. Most importantly it has the 6x7 option which I use 99% of the time. It is a considerably larger neg than 6x6 (56x56mm vs. 56x69mm). If I only (or mostly) did portraits, I think I'd be more inclined to go for the Rolleiflex, but for everything else the rangefinder wins as far as I'm concerned.
As far as lens characteristics go, I don't know much about the Rolleiflex, but the GF670 has a great lens which draws a smooth and pleasing image at the wider apertures whilst becoming razor sharp when stopped down a little. I tend to use between f/3.5-8 for portraits and f/8-16 for landscapes.