From my experience, the router bit throws/keeps most of the chips above the table, with the work piece forcing a not-insignificant quantity of debris into the plate cutout (for the router bit). So, I believe it is important to address dust both above and below the table. Also, as the Forum points-out, the router heat build-up is an important consideration - very few router table designs have a fully-enclosed router location. My router table use is not continuous; that is, I'll run the router for perhaps 5-8 minutes at a time, with perhaps 10 minutes or so between power-ups, so I'm not too concerned with the router overheating. Since each of us uses a router to different extents - some prefer a router for dados, while others use a table/miter saw; you can cut 45 degree miters with the appropriate cutter, or do this on the Table/Miter saw (personal choice) it's impossible to have a rule for overheating the router - even the amperage draw has something to do with it. Pulling fresh air from below or from the side of a router would seem to be equally effective - I don't know enough of the dynamics here. The under-router solution would tend to pull chips down 'through' the router more than a side-mount, but I'm guessing here.