I would go with the double end bag. I think you can do more with it. Both are important to improve accuracy and speed. The DE bag is the superior choice in my opinion. I feel I have developed far more in my boxing than any other apparatus--coupled with the heavy bag of course. It really works well to improve punching, footwork, bobbing and weaving, slipping a punch, and cardio.
Always get a quality leather one if you choose the DE bag.