Hi ho,
A number of years ago I had a Cman RAS built in to a 12' bench. It was a friends that he had no room for, so we set it up at my place. He moved away and so did the saw, so when house renovation time came around again, I purchased a Milwaukee 10" sliding compound miter saw. Here's the main differences I found between the two:
Cross-cutting: If you have the bench space and can build in the RAS, it's wonderful for making very accurate cross-cuts, especially when you need to take just a tad off the end off a 12' 2x10. The SCMS will make the same cut, but you need to figure out some method of supporting all that lumber that's hanging off the end of the little 2-foot wide SCMS base.
Cross-cut Dados: Again, if doing lots of repeated cross-cut dados, the depth of cut adjustment on the RAS is better than the SCMS. The SCMS can be adjusted for dado cuts, but it's tricky. Also, on the SCMS, as you raise the depth of cut from the table, you also cause the blade to not cut all the way to the fence on thicker stock. So if you're making dados in 2x stock, for example, you'd need to add some spacers between the fence and the stock to get the cut to go all the way through. Remember that the base of that SCMS is only a couple feet wide, so you need to also add those spacers out on the ends of whatever you've rigged up to support the stock. Finally for dados, with the SCMs you'll either need to make lots of little passes, or a pass along each edge of the dado, then chisel out the waste. With the RAS, you can install a dado cutting blade stack to cut the exact width you need with one pass.
Rip-cuts: Don't bother. A pair of sawhorses and a worm-drive circular saw with a rip guide will do a better job on dimensional lumber, IMO. For sheet stock or furniture lumber, use the table saw, period.
Table Wear: With the SCMS, the table spins with the saw, so regardless of the angles you're using, the saw always rides in the same slot. Eventually (if doing a lot of compound miter work), the plastic plate the blade edges rides in will get wider and wider, and need to be replaced to prevent tearout when making crosscuts. The plates are inexpensive and easy to replace. With the RAS the wood table/bench is stationary, so every time you make an angle adjustment to the saw, you're cutting a new slot in the table. Eventually the table will look like someone's run amok with a router on it and it'll need to be replaced. Just something to keep in mind when building the saw bench if you choose to go down the RAS path.
The Bottom Line: If you're planning to do framing carpentry work or roof rafter work, I'd highly recommend the SCMS. It's very portable, and if you purchase a few extras, it will do the job very well (I bought a StableMate saw stand, and additional stand extension for working up to 16' lumber, as well as a shelf and wheel accessories to make it all portable). If on the other hand you're planning to do cabinet or furniture work, I'd say get the RAS and build it in to the center of a 16' workbench. It will give you much better stock control and easy repeatable cuts, as well as greatly improved cross-cut dado capabilities.
Hope the helps,
-JeffK
