I think the main difference is the construction of the bearing cups in the hubs, with the more expensive ones being a little better. In the real world, I think all the Shimano mtb hubs perform very well. As far as which ones are sealed better, I don't think there'd be any significant difference-
Unless they've made a design change very recently, "vanilla" Deore is not equipped with the innermost lip seal (located on the bearing cone, right next to the ball bearings) that's found on Deore LX and Deore XT*. If you're building wheels to use in really harsh conditions, such as an all-weather commuting bike or a mountain bike that sees lots of mud and water, bump up to M580-series Deore LX, it's not much more money than Deore. If you find some M570-series Deore LX, they're also good.
*the latest-generation Deore XT, the 770-series models, use a new seal setup where the black outer dust cover is no longer a soft rubber contact seal anymore, more like a labyrinth shield made of plastic. Prior to that, Parallax-type Deore XT hubs such as the 760- and 750-series were set up like Deore LX.