Forge or sub

A forge is better because it has 2000 troops x 180 as of 500 x 360 forge is twice as strong

 

... but you might as well build the sub as you're gonna tear down the forge later anyways and you can always upgrade the sub later.

 

Sometimes the sub puts you in a hole where your short in $. If you can easily get another $350 mil then go ahead. Otherwise wait til the 900 mil land then start changing all