The RAS is a toned down angle grinder. Where the normal angle grinder is mostly meant for working with metal, the RAS was made less powerfull to make it more suited for the softer material that is wood. An angle grinder typically has 10.000 rpm and power ranging from 800 to 1400 watt, while the RAS has only 3800 rpm and 500 watt.
The problem with a normal angle grinder when you want to use it with wood is that it is so agressive that it can grind your paint and wood away faster than you'd like.
The difference between a RAS and a grinder depends on for what you want to use it exactly. Lots of Americans here seem to be very fond of using the RAS for scribing, and apart from the dust collection, it doesn't make much difference if you use the RAS or an angle grinder for this task.
But if you want to use the RAS for what it was meant to do, namely fast paint removal, then the RAS has a huge advantage over an angle grinder. Because the RAS behaves like a real sander, albeit a powerful one, but still a sander, and makes it easy to concentrate your efforts on the layer of paint alone. The grinder on the other hand, is so powerful and aggressive that it will eat the wood under the paint just as easily as it will eat the paint, leaving you with the difficult task to leave a smooth surface. Of course it can be done with an angle grinder if you're careful, but it actually takes a lot of effort not to mess things up. And that is simply wasted energy. Now when you want to do larger surfaces, the RAS is a lot more precise and more comfortable to work with.
