The smooth is the best finish in my opinion, but the hardest to spray with if your a can novice....
The other thing worth considering about Hammerite is that when it gets warm (even if its close to heat) it gives off fumes (and not the trippy ones).
Hammerite sent a disclaimer to all suppliers 5 or so years ago as they don't write it on the cans.
Normal Halfords stuff is better in my experience, and with the occasional promotions they do a lot cheaper!
Cheers and tata