Have you not ridden them both back to back Lucy ?
I have a smilarish dilemma since the prospect of 2 grids has evaporated.
I really like hooning around on the 583SS, as it is so easy to keep it pinned for most of the time that it _makes_ you try and get better corner speed.
That alone makes it fun for me, but I suppose it also lets me stay in my comfort zone more.
The 620 delivers the power so much more willingly that I find I have to shut off more often, and that is a challenge in itself when I'm trying to learn to be smoother in the corners.
But that makes it more of a challenge than the 583, and makes me ride faster lines more often !
In the end, at donington, I chose the bike that I thought I was more comfortable on for the race, and did actually achieve better times than I did in practice.
If you cant get both bikes to snet, but can blag some extra practice time, you can borrow my 620 if you like. Alternatively, its 25 for an extra race
(even if it is raining, you will still find out something by riding both bikes back to back !)