Have both; the Convertible Blackhawk is definitely more practical as a field gun--and a bit lighter. I bought the SS NM Vaq because it was purty. Adjustable sights are awfully useful, and there's not much downside to them.
If the NM Flattop, now a limited production Lipsey's item, becomes a regular production gun, that would be my new pick. As it is I have just about enough guns.