Theoretically the 250# scooter has 15.6# per HP and the 350# scooter has 18.4# per HP so the lighter scooter should be faster, all other factors being equal.
Example my 250 Xiciting is about as fast to 60 as my 500 Xiciting. The 500 has taller gearing so it keeps on accelerating when the 250 with the lower gearing is topped out.
I was surprised when I rode a little 50cc Kymco. It accelerated better then I thought it would but quit pulling before 30 mph. Gearing makes a big difference.