If you're in nottingham then have a word with couchcommando, he's bought/build/sold hundreds of dd bikes!
I've ridden a 620 monster for 2 seasons, and had a go on a 583ss on a track day, so can't claim to be a voice of experience on a comparison. Have to say the little ss felt like it handled better but that might just be the way it was set up. If I had to make the choice now then it'd just be down to which one was available at the right price!
Thinking more about the brakes:
Think of the effort it takes to haul the brakes on, then multiply that by how many corners and how many laps in a race. The very last corner of the race you'll still need the strength to be able to brake 100% without screaming in pain from your wrists wanting to be doing something else!
But Phil_h is right, you've got to be careful of the twin setup being a lot easier to lock up, as I've found on 2 occassions
