Having owned/ridden 125/150/250 and up to 600 scooters-----125 ridden right can be ok---just do not be afraid of running full throttle (had 125 Daelm in Korea--new--60-65 mph top--scooters can not go on Freeways there) had 2 stroke 150 Vespa in Europe, again wide open fine, 150 Vespa better than 200 in daily use.
But in real life I consider 250cc min. at least in US to keep from getting run over----had 4 new Helix 250's, 175,000 miles with them. and some where 250 Reflex thrown in. So once again prefer 250cc for serious use.
Had 2 new Suzuki Burgman 400's----heavy/performance fine.
Also 2 new Honda SilverWing 600's----over 90,000 miles with them expensive/heavy/complicated
I would say 125 performance fine,, 150 not much difference----rider skill plays a part