I hunted with a side lock for many years. In all honesty...it got to easy, so I quit. The deer haven't really been pressured, and you can (or should be able to) get very close. I have nothing against SMLs, but I wouldn't allow them in the ML season either. I just don't think either falls within the spirit of hunting with a ML. My personal opinion...it should be side lock or flintlock....and possibly patched round ball. My only reservation with round ball is on game performance, otherwise that would also be a requirement. Heck, I wouldn't even be opposed to making it smooth bore...
I just feel like these modern advancements in ML technology have moved away from the original spirit of the ML season.