When to use a model & When to use a brush

[MajesticMadman]

Hi,

So I recently took up mapping, and was trying to re-create a location from an image. To get a better grip of making an overall level. So the image below is what I'm using:

 

 

So the plants & arch will be models but I wasn't sure what best practice would be for the "pod" like things and the water tower. Would they be better modelled or made with brushes. And how would you determine which goes where? 
Should anything not a wall or floor or ceiling be a model?
 

[Noodle]

I'd make all of them models to be honest, given that their forms are highly irregular and some of the details would be missed with simple brushes. 

[AshuraDX]

I'd make all of them models to be honest, given that their forms are highly irregular and some of the details would be missed with simple brushes. 

I'd agree with this.