I have recently bought new rear freehub Shimano Deore for disc breaks as replacement for stock Author freehub not with disc mounting.
But that Shimano's hub hasn't rubber seal as Author's has, but it has some metal seal ring instead.
My question is: Is that significant difference between rubber seal and metal ring seal? Why is it still on the market, since I saw that almost every hub sold today has rubber sealing?
All Shimano hubs I've seen use a labyrinth seal design. The parts that protect against bearing contamination are actually the (metal/rubber) parts that present a non-direct path for contaminants. It's not the material but the design that should interest you, and in this respect, Shimano delivers. Unless you submerge and run the hub in water, labyrinth seals are allegedly better than those rubber covers on cartridge bearings (that I presume is what you refer to as "rubber sealing").