The main differences are exactly that, aesthetics. LMDE also includes some nice Mint-developed programs to make administration and backups etc easier.
As for updates, it is up to you. LMDE works with "Update packs" which are released on a regular basis and update a number of packages. It is, like Debian, a rolling distribution (they call themselves "semi-rolling" because of the update packs) with no real versions. However, it is completely compatible with the Debian repositories and is, in all ways, Debian so you can simply point it to the
experimental repo to get bleeding edge.
By default, LMDE is based on Debian Testing, and is essentially Debian Testing + Cool Mint Tools. The way it is updated depends on how you set it up. If you want stability leave it at the default state which will point to the Testing repositories and Mint's own "Latest" repo. To get the latest updates as they come in, point your
sources.list to Debian Experimental or LMDE "Incoming".