Is there a difference in sound between the two?
Well, I only have one of each, so not really that experienced with them. But I prefer the Marine Band. I do think it sounds a bit better, but that may just be from the cover vents. I'm not sure how much someone else listening would notice, but it sounds better to me.
I also think it's a bit more responsive, though my Blues Harp is a better than the other MS harps I've tried (Big River, Pro Harp, and Cross Harp).