while not all mechs are built the same, there's not that much difference as far as how well they allow you to "chuck clouds". That comes down to the coil, the airflow, and the power being applied to the coil. A mech made of pure silver will conduct electricity better (and thus provide a touch more power) than one made of copper or aluminium, but unless you get one custom made by a silversmith. . . Mechs just provide a means to connect the battery to the RDA, so it's really going to be down to what battery they hold and what battery you use (one that's only 10A Vs one that's 30A, etc).
Aside from what battery they use, I'd shoot for copper connections, comfortable to hold and use, easy to carry, and reliable (don't need one whose button is going to fall apart).
You may know all this, but since I read your post as 'wanting a mech that chucks clouds', thought I'd mention it. I've seen sales clerks tell people that such-and-such mech makes bigger clouds than the others, when really it was about which one the sales clerk wanted to push.
