Well I would like the machine to be able to turn the max diameter that can be put in the chuck. I will be working with steel and aluminum.
It has been suggested that I go with the Grizzly 0602 model because it maintains torque at lower RPM.
Well, the stock chucks are 6", and the other poster mentioned running that. Depth of cut matters, but what you're getting at is speeds and feeds. Larger diameter needs lower speeds to maintain the proper rates. That's somewhat tricky on a lathe, for facing anyway, as the correct speed varies based on the diameter of the work at the tool's location.
The fully variable speed models are nice here as you can adjust on the fly while the power feed handles the cut. It also depends on the material. Aluminum is fine, you can run that at such high SFM that you don't need to really worry. 12L14 steel would be fine. 4140 would need carbide, but you can get it done. For a low carbon steel like 1018, at 6" diameter, HSS would want to run about 70RPM. That's low enough you might see some torque loss. Switch to inserted carbide and you can move up to about 350RPM, which would eliminate that concern.
The grizzly you mentioned uses step pulleys for speed adjustment. That's where the mention of not losing torque comes from. Speeds are less flexible, but you don't get VFD loss. In practice it isn't as big an issue as some people think. Particularly with modern motors. And if you use the right tools for the job.
While testing some changes, I took 0.250 off the diameter of a 12L14 rod that was about 0.280 at the time. HSS tool, 350RPM, using power feed. No chatter. That was partly due to swapping the compound out for a solid block, but it gives you an idea. That really hard steel I mentioned, I can get about 0.100 depth of cut with good finish. That requires nicer inserts though.