I am not an expert on the matter but here is what I know:
Tamiya has both the King Tiger Porsche and Henschel Models.
Tamiya kits have very good quality as far as details and molding are concerned. The kits come with vinyl tracks.
Dragon kits also come with good details and the molding has greatly improved compared to their previous releases. Dragon included PE parts for their kits as well as individual track links.
Between the 2, Dragon kits are cheaper than Tamiya ones.
I have built the Tamiya King Tiger Henschel Turret and it is a very good kit. But upon seeing Dragon's version, I think I'll get Dragon next time I decide to build a King Tiger.