Eric, getting in on this discussion a little late, but on my router I've gone for a fixed gantry and moved the table. Pretty damn rigid and a more than capable machine - despite me building it!! Only limitation is the spindle. I made my own spindle using a er16 chuck, and out runner motor. It runs well on plastic and wood but stalls easily when taking any sort of a cut in brass or ally. If I want to cut brass or ally, it takes ages because I have to take such small cuts! That said, I've done some lovely engraving and cut out some nice brass parts up to now.
Do you plan on using Linux CNC or mach3? Despite having a little experience with Debian, I sucked with linuxcnc! I just couldn't get it to work properly. Installed mach3 and all my troubles went away!
I don't know what you are intending to actually cut, but I couldn't ever recommend V-carve highly enough. Truly amazing. Powerful and once you get a grip of how it works, v easy to use.
Keep us posted. Immediately when I made my small CNC, I got a load of extrusion to build a bigger machine! It is all here waiting, so if you do use ally extrusion, plenty of pics!!! - I might copy you!
Chris