One of the online retailers has these three frames on sale for roughly the same price (about 2100). The Merckx and Pinarello each weigh around 2.85 pounds according, and the Opera about 2.3 pounds, according to the website.
Other than that, I don't know much about the frames other than the geometries, and the fact that the Merckx is the only one made with Scandium tubing.
Which would you choose and why?!

All great frames I'm sure; I think the Merckx is personally a little more classic/sexy, and you really can't go wrong with the bike that basically took Paris-Roubaix and a hell of a lot of other races in the pro peloton.

They are all great. Usual ride-em and decide advice I guess, but purely on aesthetics, I'd take the Merckx. Some of the new Pinarello and Opera styling is a little bleh to me I guess.