I've got a DeWalt 12" SCMS and a Craftsman RAS and I like 'em both, but I've got some room in the shop. First of all, the portability of a 12" SCMS is overstated because I've had tablesaws (BT3000) that were more portable. I keep a 10 CMS for portability. The RAS is used by me for cross cuts and dado's, nothing else works as well for dado's, and I like the width of cut capability. The SCMS makes good cross cuts (out to 12"), is easier to set up for miters and has the compound capability.
They are completely different form one another (the SCMS compares with a CMS not a RAS), if you don't need a deep crosscut, dado much and have a tablesaw, then you don't need an RAS. But definitely don't think that a SCMS or a sled for the tablesaw can do what an RAS can do. It's kind if like comparing an orange to a grapefruit... They are both citrus fruits, and are used for breakfast juice drinks but one is much larger than the other and the taste and uses are completely different.
If you don't dado much and need the deeper cutting capacity, then get rid of the RAS.
Good luck Rob !