I'm not sure either is really "better" at cleaning fish. In reality, a QT tank should be ridding your fish of dieases and parasites before they enter your tank. No cleaning animal can keep a tank 100% clean of diseases and parasites. So go with whichever you like better.
The clams like sand. As long as you have a sand bed for them to burrow into you're fine. Just be warned, you'll never see them again.