Long day at work...
I believe that
@jwmelvin highlighted most of what I regard as more rigidity. When I owned my PM833T, the power of the machine was never an issue. It would cut what I needed to cut without too much complaint. However, vibration and chatter were sometimes apparent, especially when the gibs weren't snug or the Z-axis wasn't locked down. Of course, locking and unlocking the Z-axis inevitably introduced errors to the setup, which was frustrating. The 833T does weigh near 1000 lbs, so it's not exactly a "light" benchtop mill.
Contrast that to my Sharp mill that weighs somewhere around 3500lbs. Mine has a 2-axis CNC system, so it is closer to 4000 lbs. The difference was astounding when I installed an end mill and made my first cut on the Sharp knee mill. It is hard to quantify in a post on the internet, but I knew right away that I had made a good decision. Smooth. Precise. Quieter. Robust. It is just a world of difference IMHO. That being said, my mill has some wear and there are a few things I need to fix, but now that I know what it is like to have that mass, I wouldn't go back.
Just my 1.5 cents (inflation ya know)
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