For bubble counters, I've found that simplest is best. The mounted versions typically have several parts that screw together, and an integrated check valve. Each joint can leak. And the integrated check valves are garbage. And when (not if) the check valve fails, you won't know it, because there's no way to tell that you're losing fluid back down into the needle valve and/or solenoid. All check valves will eventually fail, but those silver ones are almost as bad as the plastic local pet store variety. I like the simple, one-piece, three-dollar glass inline bubble counters from Evilbay.
Diffusers will always be hit or miss. The cheap glass and ceramic guys may or may not work well. I've had success in using superglue around the outer rim of the disc (often a problem area with too-large bubbles). If you use those, buy 2 or 3 at a time, and test them all. The bazooka-type referenced above (just search for "Atomic" or "Atomizer" diffuser on Evilbay) do tend to work much better. While I'd tentatively agree that the inline variety work better than the in-tank variety, they're more difficult to clean, and they *do* need to be cleaned periodically. A simple bleach and water dip once a month will do for the in-tank variety. Cleaning the inline variety is more complicated and possibly less effective. You really don't have to spend ridiculous amounts of money on the in-tank variety. I think they're as low as $10 on Evilbay, and I don't believe there's a definitive quality difference between the more and less expensive varieties. They all, originally, come from China.
Bump: By the way, the Atomizer (tube-variety) diffusers require a higher pressure to function. I've used maybe half a dozen, and they have required between 25 and 40+ psi to function.