I tried both 250 and 300 at Inch Perfect test day. As stated above and confirmed by John Lampkin the UK 300 is to SS spec.
I found the best way to assess them was to forget engine size and just focus on how well they rode for me. In doing this, I (old timer clubman rider) found that I couldn't really decide between them. Both were soft and forgiving, no sense of the 300 being too much bike. I did find, though, that the 300 needed more beef to kick start it, perhaps a factor for a not particularly fit rider during a tiring trial.