The Kzinti dreadnought is a resin body with metal engines.
The Klingon C8 dreadnought is a one piece, resin body with metal engines.
The Romulan Condor is a one-piece, resin mini.
All others are now in metal, including the Federation and Gorn dreadnoughts.
Not sure about the new battleships coming out, but I'd guess they follow the same path as the dreadnoughts.
The Federation BB is metal.
The Gorn BB will probably be metal.
Romulan BB wil probably be resin (and one piece).
The Klignon and Kzinti BBs will probably be resin bodies with metal engines.
But, that's just a guess - and i have been wrong before.