I have a One model 250,from the first year units, I tested the 300 and is has a stronger pull but not brutal at all.
If you are afraid... I would suggest to test a 125, you may be surprised how easy is to control it off idle if your experience is not high, a friend that rides with me comes with a Fantic 240, (is 212 cc), I recommended to him to get a modern 250 and he bought a gas gas, fantastic bike, he never got user to it, the bike reacted to fast for him so after 6 months he sold it and went back to the Fantic, last winter, we push him to test a very special bike, with lots of work on it, it was again a Gas Gas but 125, we kept secret it was a 125, he tested the bike a full morning and love it.!
I choose 250 on all bikes I have bough during the last 25 years, it is the best blend of softness and performance for my level riding.