This is a difficult decision. There are pro and cons for both. With milling machines weight is everything. Larger castings allow for a more rigid setup, which decreases tool chatter (deeper cuts can be taken and better surface finish can be achieved). The Pm-833TV is twice the weight:
Pm-833TV - 750 lbs
728VT - 370 lbs
However, it’s important for you to ask yourself what are you going to be using this CNC mill for? Is this a hobby where you want to learn how to machine and program? Or will you be using this for small production runs? Is there a certain size part that you have to make?
If this is just a hobby and you are new to the game, then I recommend the smaller machine -728VT. Two burly men could probably pick it up, making it much easier to disassemble and work with. Also, its 30% cheaper. That’s nothing to scoff at. At least in my area, hobby level CNC mills have great resell value, so if you need a bigger machine then you can sell it and upgrade. However, many people go from hobby level mills to a haas (or comparable brand) because they want more sophisticated features like ATC, through spindle coolant, chip augurs, etc.
If you have some experience with milling, a lot of time to kill, or the need for higher quality and deeper cuts, then go with the bigger machine!
Let me know if you have any more questions!