Hi,
I have some of both types of tables, the big advantage I see to the off-set design is if you have the router mounted on a removable plate and this lets you have some place to sit the router down while you change bits etc. If the table is big enough this become less of an issue.
If you do put a miter slot in you might want to get an aluminum miter slot extrusion or better yet one with a t-slot. These can be glued/screwed and glued into the table and will more then make up for the missing material when the table is 1-1/8" thick or more.
I don't use a miter gauge very often but do like to attach finger boards and other jigs using this method. If you have trouble finding these let me know I have a lot of catalogs laying around and can get you started with some examples.
Ed