I'm not a big fan of Metal Casting this early unless it's from the Oracle. It's a very expensive tech at this point in the game and opens only 2 things: Forges and The Colossus.
Forges, while awesome, are a huge hammer investment if you get Metal Casting early (and are not industrious). They also tend to come at a time when there is a lot of other infrastructure that needs to be built - Granaries, libraries, temples, courthouse, lighthouse - in addition to the units you want - workers/settlers, fogbusters, an army to conquer with. The trouble is that you start thinking about how everything else gets cheaper once you have a forge, but they cost 120 hammers which takes a substantial number of turns to build. When you have so few cities (2-4) it's a real strain to lock one up for 15+ turns at that stage.
In short, unless you're industrious and/or there are gold/silver/gems to get the happiness from, forges are too expensive for that stage of the game.
The Colossus is okay but really requires a strategy dedicated to using it. I don't think that's the case this game.
Typically my neighbors won't have enough techs that make trading MC worthwhile at that point (no fair or close enough trades) but often aren't shy about begging for MC for free, which usually results in a "you're kidding, right?" look from me and a demerit.