My kid has a Ninja 250 in the garage too. If you want to keep it more than a year, go for the 300 unless you really want a 23-27 RWHP and the endless shifting, downshifting and near zero torque. That was my point about the 300, you can lug it and roll on the throttle and it actually accelerates. The 250s need to be in exactly the right gear all the time. You can ride the 300 like a "normal" street bike.
2 up is torture, as is long up hills. No so with the 300.