I've been ranting for years that Dremel has consistently missed the boat in designing rotary tools for modelers. Whenever a new Mototool gets introduced, the company brags about how powerful it is and how fast it turns. The truth of the matter is that, for at least 90% of modeling purposes, the cheapest Mototools on the market are more than powerful enough, and, up to a point, slower is better.
I've got a relatively new, ball-bearing Mototool with a built-in speed control that lets me dial in speeds from 5,000 to (I think) 30,000 rpm. At 5,000 rpm a brand new, sharp drill bit will drill styrene or wood for a while without melting or burning it. But as soon as the bit gets even slightly dull, the results become disastrous.
I got my first Dremel Mototool about 35 years ago; it was a prize in a contest, and I thought I'd gone to modelbuilding heaven when I opened the box. When I got it home and tried it out, though, I discovered that it was virtually useless for my purposes. It had only one speed (it was, in fact, one of the cheaper versions of the era), which made it hopeless for working on plastic and wood.
So I went down to the hobby shop and, for the staggering sum of about $15.00, bought a Dremel "Tabletop Speed Controller." What a difference. The tool plugged into the speed controller, which had a knob and a switch on the front panel. With the tool and the controller turned on but the knob turned all the way to the left, the tool would sit still. I could put the tip of the drill (or grinding tip, or whatever) exactly where I wanted it, then slowly turn the knob and ramp up the speed from zero to whatever I wanted it to be. I still contend that's the easiest and most reliable way to use a rotary tool on a model.
Unfortunately Dremel quit selling the "Tabletop Speed Controller" quite a few years ago. Several similar gadgets from other companies are on the market nowadays, though they're pretty expensive. (I imagine any speed controller designed for a router would work - this one, for instance: http://www.woodcraft.com/family.aspx?FamilyID=3957 .) I eventually bought two of the old original Dremel versions, and I still have them; they work as well as ever. They come in handy for all sorts of stuff around the workshop. I keep my 30-year-old Unimat lathe/drill press plugged into one of them, so I don't have to swap pulleys on it to change speeds. A Dremel Tabletop Speed Controller will control the heat of a soldering iron. It can even change the size of the picture on a black-and-white TV set. (Don't ask me how I found that out.)
My current favorite rotary tool for most modeling purposes is this one: http://www.woodcraft.com/family.aspx?FamilyID=3736 . It's slightly thicker than a fat pencil, it runs smoothly and quietly, it has a flexible cord that comes out the back end and doesn't interfere with the work, and it costs $25.00. It doesn't have a speed control, but it works beautifully with my old Dremel ones (and, presumably, with their modern equivalents). Though the dealer doesn't mention the fact, this little We-cheer tool accepts Dremel collets. That means it will hold anything from a no. 80 drill bit up to 1/8". I haven't used it enough to make any pronouncements about its durability, but I figure at that price if it only lasts a few years the $25.00 will have been well spent. Highly recommended.