Mine is corded, but I think I need to invest in a cordless. Twice this summer at the track I noticed I had stripped screws and I had to go ask the guy who brings a dremel and a generator if I could borrow it.
For a primary dremel, I would say corded is the only way to go, for a travel, or light use, cordless should work. If I have any issues with the cordless (once I buy it) I will let you know.