One of the trainers that we did classes with was a big believer in the martingale collar. She was English and very into positive training. I did try one on Baxter when he was a bit older...probably around a year old, and not all that impressed.
How much does Arthur weigh now? about 40 lbs? He's getting stronger and pulling harder now? The martingale may give you what you need to get a bit more control of him now. Make sure it's fitted properly too, I've noticed people have them on too loose and they tend to slip off.
I honestly don't think they give enough control for a full grown Rottweiler.
What does your trainer say? what do they prefer you use?
Gina