I used to work for the company that makes Dremel brand rotary tool products, back in the 1990's. At that time, Ryobi came out with a competing corded tool. Instead of a universal motor with replaceable brushes (similar to a Pullmor) it has a PMDC (can motor) and electronics. The electronics included a feedback mechanism such that as the tool worked harder, it supplies more current (phase?) to the motor. The upshot is you could put it at the lowest speed setting so it doesn't throw sparks, then start cutting. It had a dial near the cord to select speed.
Dremel decided not to add speed control to their product, but out maneuver them in marketing.
Fast forward to 2018. I see Ryobi still makes it, the HT20VSK. Dremel might have changed. Dremel might make a PMDC motor tool, might have one with feedback, and there are other competitors. Also back in the 1990's NiCAD batteries were only good for about five years. Can't speak for NiMH or Lithium. I would venture to guess if you are going to use it alot, go for with batteries, and if you want to be able to use it in twenty years, go with corded.
I have both that Ryobi and the Dremel rotary tool from 1990's, both still work, so no need for me to go shopping or looking.
We were taught that the word "Dremel" is like "Dumpster" or "Kleenex", it is a brand name, and employees had to make sure not to use it in a generic way.
Another thing I learned was the markup on accessories was way more than the tool. I remember being in a meeting with the president of the company that could be summarized as "If you are in the way of our company not getting enough accessories to our retailers, you might as well pack your bags right now." Became the #1 priority that day.