A lot less shifting with a CRF250 but they aren’t as fast as a CR125. I think the HP is about the same but the CRF is heavier and it’s hard to get the kart balanced. The maintenance IMO is a tos up. The CRF is more complex to re build but they go longer than the CR’s and parts are everywhere.
The CRF 450 (around 12 lbs heavier than the CRF 250) will kill a 125 anything.