This is hard choice to make. Frankly you can get fooled ball screws and nuts that are very competitive with common lead screws. They might not be the highest quality when it comes to lead accuracy but they are very capable for many machine applications. You imply that you are already running a shop with plenty of machine tools, this leads me to suspect that you might have higher expectations than many for this machine, which leads to more expensive or accurate parts for the drives.
Personally I'd go with ball screws and use an automatic lube system to help keep the ball races flushed out. As you note dust is a real problem with ball screws so you would want to address that. In this sort of application I don't see ball screws as being remarkably better but simply have a preference for them over ACME or similar lead screws. Part of the reasoning here is that I see you getting better high speed performance which starts to be a real issue when machine stroke lengths are beyond about a half meter. In other words rapids can be very important with the longer axises and as such ball screws should perform better.
One thing to watch out for is nut heating with synthetic lead screw nuts. Take this advice with a bit of salt as there are many new materials available that we didn't have when I first saw this problem on machines. What we ran into with was heating of the nut & screw, causing tolerances to close up and thus leading to high torque on the leadscrew shaft. This was in a high production environment trying to hold very tight tolerances on repeatability (1 to 2 microns) in a precision lathe. So getting things adjusted just right was a challenge.
Of course everything heats up once the machine starts running but generally you don't have ball screws heating up to the point that the motor/servo drive faults due to following error. For a light duty, small router you likely could flip a coin and get good results either way, that is ball screws or lead screws.