Ideally, you want something with multiple speeds to choose from. When I use a rotary tool on plastic, my strategy is to go a head and let it eat up the material. Yeah, it's messy, but, with a little bit of practice, it's not so hard to figure out how to flare out the material just the way you want where the raggedy edge can easily be broken off with your thumb and finished up nicely with some sharp blade or judicious use of sand paper or the right shaped little file.
Normally I'll use those little brown cut-off discs. I don't know what their made of, but it looks like basically compressed grit, almost like a thin, fragile ceramic. If you need to cut a wider trench with it, you can stack a few together. These little bastards break if you look at them wrong, but that's okay as they usually come in packs of 30 to 100. They're meant to be pretty disposable. This really is my "go to" tool when working plastic. Don't bother with carbide burrs or diamond bits or ceramic stones or any of that stuff. They're great for other materials, like soft metals or ceramics but they area mess with plastic.
(I'm actually a dental technician by trade and I spend all day every day using a rotary tool on gold and porcelain, plus I've been rearranging Star Trek
model kit parts since I was about 12 years old, so I have a little
experience with this stuff.