A vari speed does not limit power or the mechanical speed ratio vs a pulley drive. A Vari drive just allows continuous adjustment of both pulleys, i.e. as you make the adjustment one pulley squeezes together while the other expands. It allows specific RPM control on the fly without stopping the machine, releasing the motor and having to change belts. It requires a heavier belt and a bit more complexity and to some degree other wear parts. The vari-speed tend to have a much higher top end RPM. They have been used for decades and are very well proven. Unless you are planning to go with a VFD, highly recommended to go with a Vari speed. Between the PM 9x50 and 9x49, be aware that the latter is made in Taiwan, the former is of Chinese manufacturer. If given the choice, I would go with the 949 at this spend level (but it is 1K more), but both will get the job done. Both of the Grizzly machines seem to be of Chinese manufacture, but look like nice machines. There are a number of options at this price level.
Should have added this, both pulley and Vari speed have back gears that set the low end speed range. On the Vari Speed head there are two windows which give the ~speed based on if you are in back gear or not. A pulley drive will have the same relative ratios, just reduced by a factor of something like 8 or 10 when in back gear. Some mills (like the 949TS pulley version) have a 2 speed motor, so you have a wider speed range (or speeds) which is more attractive if you go that route.
