VFD will be much more expensive in this situation, yes motor size. Plus you have to derate the motor by up to 50% depending on the VFD. If you run an inverter you do not need a vfd and if this were for a press then a VFD is not needed. Plus the rotary can be used to run the whole shop if you want. It is not as easy to run the whole shop with a VFD. For a few $$ more you can purchase a CNC rated rotary and really step up. But that is a whole new can of worms.