Ballscrews are typically more rigid and more accurate, but cost more money. Length is limited.
Racks are typically very well suited for most motion systems and can be accurate enough for a lot of applications - including wood routers. Rigidity is more than adequate for wood. Many, many commercial machines use racks - including some very expensive machines. They allow far greater distances to be covered than ballscrews.
I intend to build a 5'X10' router in the next 2-5 years. When I build this, I will be using a rack and pinion for the X and Y.
Scott
Consistency is a good thing....unless you're consistently an idiot.