When you're underwater, you want to feel free. Having lots of cables and hoses protruding from your body hinders that a bit. That's why I recommend the SS1 combination inflator/octopus from Atomic Aquatics.

It attaches easily to virtually any BCD with the included adapter kit. The ellipse shaped design allows it to easily rest on your chest when not in use, but makes it a cinch to grab firmly when you need to inflate/deflate your vest or breathe from it. It's extremely high quality, and you will not find any problems breathing from it, even at depth (100-130 feet or so).

Just remember to tell your dive buddy/partner that, in the event of an emergency, they should grab the regulator in your mouth and use it (preferably asking first). Then, you switch to the SS1 for breathing and buoyancy control.

Oh, and get it in red. SO much cooler looking.