They are not the same in several ways....
The sliding miter saw and the RAS have about the same crosscutting capacity, more or less about 15". A 12" RAS and a 12" sliding miter saw, would compare to a 10" RAS and a 10" RAS. within an inch or 2" in capacity. I have both, a 12" RAS and a 12" sliding miter saw, and a 10" RAS and 10 sliding miter saws. That said ....
I use my sliding miter saws more for "on site" work because they are easier to move around AND because the sliding miter saw can be easily changed to miter at any angle with no loss in accuracy. The RAS stays set at 90 degrees in the shop and is best for cross cutting anything from a 10 ft long board to a 3/4" X 6" X 12" piece. The cutoff pieces do not fall into the hole like on a table saw, BUT the best solution to that is a zero clearance throat plate. No table saw should be without one! Having said that ......
The cost of an older Craftsman RAS is very reasonable compared to a sliding miter saw.
You can make dados on a RAS, but not on a sliding miter saw...easily. Some have a depth stop and you can make multiple cross cuts to accomplish a few dados if you need to do that.
The RAS blade can be tilted for bevels, it can be angled for miters, it can be rotated so the blade is horizontal, it can be used with a molding head or a dado head in any position. It is more versatile, but requires more attention to set up. Some RAS do not hold their positions well and may have worn carriage bearings or be out of adjustment. That issue is easily fixed, adjust it and replace the bearings. If the bearings are indeed "worn out" the saw was not well maintain OR used in situations where abnormal wear and tear abused it.
The Danger Factor of the RAS, always comes up in these discussions. If you have the replacement Safety Recall Guard installed, the blade is enclosed for the most part and is a safe as any type of saw. If you still have the original factor blade guard, they work fine when properly adjusted. Safety comes into play when working with very small pieces and the safest way for cutting those is on a table saw with a cross cut sled OR a miter gauge fitted with an extension. An exposed blade is always a danger no matter what type of saw you use, you just have to keep your fingers clear and stay focused on where they are and what you are doing.
Ripping is done best and most safely on the table saw with a push stick and a proper riving knife or splitter. Kickback is a less reported injury than amputations, but it can be avoided almost entirely with a splitter or riving knife. Ripping on a RAS, is best left to very experienced woodworkers who know how to adjust the blade guard to hold the work flat on the table and who use a push stick when approaching the blade. Ripping is not possible on a sliding miter saw UNLESS the pieces are 15" or less in length, and I don't recommend it.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 09-19-2016 at 04:59 PM.