IMO, nothing. I thought there were different diameters between ZUMI and Rocket Dog of Vermont, but I have learned they're the same. I have a RDoV lead and have used Meagan's ZUMI, I saw no difference/preference. They both make dual leads which is what I like, but have different color options.
MEAGAN, I do remember them having r/w/b before! I had seen them at RDoV once too and then couldn't find them again, should probably check back there.