I've seen them both play and I don't really even think this is close. Sheppard simply does not have the on-ice wherewithal be ranked ahead of Mueller. IMHO Sheppard will play in the NHL, and play OK. But Mueller is going to be a #1 centreman and a fixture on the highlight reels.
Mueller was picked higher and pretty much everything I've read has him ranked higher. I think they will both be excellent players but I take Mueller quite easily. Maybe a homer pick to some degree but I'm thrilled he will be a Coyote.
As for the comment about work ethic, Constantine seems quite pleased with Mueller's and he doesn't seem to be the kind of guy that's easily satisfied.
p.s. Sheppard on Mueller's wing in a few years would be fun to watch.