IMO, if you've got a requirement for having a rigid, accurate machine then both the moving gantry or moving table designs give ample opportunity for problems
Personally, I like the idea of having a moving gantry and a fixed table mainly because I can picture down the road wanting to swap out features on the table (like a vacuum system, clamping fixtures, etc) and it seems to me that it would be easier to do that sort of thing when dealing with a 'passive' table.
Also, on a larger router the idea of accelerating and decelerating a big, heavy table with another couple hundred pounds of wood or metal on top isn't too appealing.
Finally, as ger21 stated, getting the cutting envelope covered by a moving table on the x-axis means that you need a set of linear rails that are 2x the length of the table. For anything over a 24" table, the cost of those rails (even on eBay) is going to be tremendous. Plus the cost of the ballscrew.
-Chris