Hey rreiner! I've got a North 6 kilo that I've been roasting on for about 4 or 5 months, and I used to roast for a company that had a USRC 12 kilo.
I was also looking at USRC and Mill City before I purchased this machine. I knew that I liked USRC but I was curious about the North machines (same as you, due to budget). After spending an hour or so on the phone with Steve at MC, I was really impressed. (As a matter of fact, I think I visited HB to see if I could find any feedback from others who had dealt with Mill City or North machines, and all I found were rave reviews). There's a reason why. They know what they're doing up there.
The machine itself is great. It's heavy as hell, which is awesome (as long as you have 5 or 6 people to help you move it in). The cyclone is huge, probably the same size as the one on the USRC 12 kilo I was used to. We choked down the outlet on the cyclone from an 8" to a 6" and it still doesn't get very hot - and I'm using single wall duct (running straight out of a brick wall). The cooling tray is separate from the machine, which was interesting, but the only difference is it makes it lighter and easier to install. Steve sends an extra set of orifices for NG conversion, which I thought I wanted (I was used to it from the USRC), but I couldn't seem to get enough heat out of the gas. I think the bottleneck may be my feed line, but I've been so happy with propane since I switched it back that I'm just gonna stay here. I've never had any tipping or charring.
Batch sizes - I've found a sweet spot with a 9 - 10 lb charge. I've done a little bigger (11.5 or so) and as small as I think 5lb. They all work if you know what you're doing with your air and gas, but I stick with a 10 lb charge and have all the control I could want.
Any other questions, ask away!