From what I've seen, the WARN standard hubs are built with some plastic internal parts. The premium hubs I've dealt with on the other hand are composed of metal. Also, when locking/unlocking the hubs, the standards only require about a quarter turn of the dial, where the premiums allow for almost a full rotation (I like this feature).
As far as if the premium is worth the extra expense, I will say I've always been happy with the premiums which is what I've always used...never tried the standards to give you a comparison. The premium hubs also carry a lifetime guarantee against mechanical failure. For all I know, the standard hubs may also have this same warranty, I'm just not sure. Either way, I don't think you can go wrong with either version, I just have had first-hand experience with the premiums.
--Digger--
http://www.users.qwest.net/~ascanlon