The cordless ones run slower and have much less torque. If you're working in plastic it won't affect you much, but if you're going to use it on wood or metal (and you will, eventually) you'll find it lacking. I have both, and only use the cordless one for real quick jobs away from my workbench where I don't want to pull the corded one down from it's hanger.
In addition to the flex-shaft attachment and a jawed chuck, I recommend getting a variable speed switch pedal. You can get them cheap from places like Micro-Mark.