Hi Patrick,
My last bench top mill had a 3Hp BLDC variable speed motor with a 3 speed gearbox to cover a speed range of 50-3000RPM. Variable speed allows you to change speed on the fly and dial in a specific speed for the work conditions. It makes it much easier to get the optimal speed and feed. Trying to achieve this with just a 6 speed gearbox w/o variable speed is cumbersome and the speed steps are very large. Given the choice and nominal cost difference, variable speed is the way to go. You need the six speed gearbox to give you the power band for each variable speed range which I would estimate would be 40-100%. As you decrease the variable speed down you loose Hp.
The main reason for a belt conversion is noise (which I have not heard as an issue with the 727V) and increase spindle speed. You are getting a mill with a 1 Hp motor, so you need the gearing steps or multiple pulley steps to provide the mechanical ratio within the operating range of the variable speed motor. Most people that convert to belt drive use two steps and use a larger motor, otherwise the performance will suffer. Conversion to a belt drive is usually done on conversion to a CNC machine where you they are spinning the spindle at much higher speeds using a small bit. Converting a PM-727V to a belt drive would compromise the machine significantly if using it only for manual milling. You would also be spending a lot more money unnecessarily converting a PM-727V to belt drive vs. buying a PM-30MV which comes with a 2 speed belt drive with a 2Hp motor.
Both the PM-727V and PM-30MV, offer an optimal milling package in my opinion for their size and price. If you need a smaller footprint and/or only have 120V go with the PM-727V, if you want belt drive and have 220V go with the PM-30MV. Size matters, but as far as travel (X, Y, Z) the 30MV gives a bit more X travel, but otherwise they are very similar in this category. If you have 220V, the PM-30MV would be my recommendation. Be aware that the cost of equipping your mill (vise, chucks, end mills, etc.) can easily be the same cost as the mill itself.
Mark