Wood.... there's not much I wouldn't trade to have access to a 16x24; that seems to be a ballpark size for your Mk1 american double garage. That said, that 8' ceiling seems a little too low for me; if you intend to work with fill sheets of material it could become a prob. Additionally, more often than not you'll find that rough sawn boards tend to be in the 9-12ft bracket. Given the choice, I'd opt for a height of 10ft at the eaves.