I am with BigJim…. owned several of both and currently have a 12" RAS and a 12" slide CMS.
Both have their areas where one is better than the other (and not as good as the other).
Although I cannot really say why, but I do use the slider more than the RAS.
Radial arm saws are famous for being difficult to reset the arm after changing it from 90 degrees and back or to different angles like 45 degrees, however there is a solution! A miter jig made from a 90 degree corner and a bottom base, held against the fence will make perfect 45 degree miters.
Not quite as I described but close:
This one is as I described:
The sliding arm miter saws are known for better return to position accuracy than RAS's and are lighter to carry to your work site. They have a slightly different operational process in that you can push down as well as slide the blade back and forth when making your cuts. I'm not sure how important that is to accuracy, however.
I love my Craftsman 12" RAS, which I leave set to 90 degrees at all times. I just set it and forget it. When I need to make miters I'll either use my Incra miter gauge on the table saw or dust off the sliding compound mitersaw, a huge 12" Dewalt 708. Here's my extended table on the Craftsman 12" RAS:
However, there is another option for me, and that is a large crosscut adjustable miter sled:
So, it's not always a choice of one or the other. If you are making "tons" of frames with miters then it's worth making the sled if you don't want to spend the money on a sliding arm saw. RAS's you can get really cheap on Craig's List because they take up a large space and folks don't understand how to use them safely. I own about 5 of them I purchased off the "list". They are an interesting machine, but I've got them figured out and found that the carriages are inter-changeable! So, I have one carriage all set up with a dado for 3/4" shelves, and another with the standard crosscut blade a 60 tooth Diablo blade. For a big bookshelf project I just switch to the dado carriage and make all my grooves:
Here's a router carriage I made for my RAS that allows the router to make dados or "T" slots and other operations:
There you go, a full blown explanation of the pros and cons .....