I borrowed a big *** single speed(f**kin fast) Makita like RAMM posted and it scared the bejeezus outta me. Needed to be able to fluctuate the rpm, so I use pneumatic. Control at your fingertips.
I'm a newbie to this, just my experience.
I only use short cutters in the Makita and I hold on to it and LEAN on it HARD for heavy and fast stock removal. I am also blessed with an above average grip strength and I can stall it out anytime I want. When I get tired I move to an air grinder and recover a bit before going back to the Makita.
You simply cannot stick a 6" shank bit in an electric die grinder--it will bend. With some skill you can start it against the port wall so you can keep it from achieving full rpm while under load--but this is not for a beginner. I have bent a few in my time--I cut em short and use them. J.Rob