[blykins]

For what it's for, I would just go with a double roller. I did for my 425 SBF. Cloyes offers a really nice chain (hex-adjust) with several different keyways for various cam-timing options. Very nice.

You may get a little horsepower increase with a gear drive, as chains are the most inefficient ways of coupling. But for a high-performance street engine, a chain is perfectly fine. You would probably get a lot longer life with a gear drive...but you probably wouldn't notice it until you hit 100k miles or so.

Also, most gear drives are noisy. I think you can buy quiet ones now, but the regular ones sound like roots-type blowers.

As for mechanical vs. hydraulic, I have no idea what you're talking about. Can you clarify?