Hi Tom
As Ric has said, there's numerous differences between the Multiple domain accounts and the Reseller accounts. There's two markets there, so the MD and DR accounts target each market.
MD accounts don't come with personalised nameservers, as do the DR accounts. The MD accounts are also domain number limited, whereas the Reseller accounts are not. The reseller accounts also come with ClientExec, whereas the MD accounts do not.
Hope that clears things up.