Do metallic pads squeal more than non-metallic ones?

Absolutely not. This is a vicious rumor spread by a minority of disgruntled non-metallic pad zealots. That squealing you hear coming from your brakes is caused by vibrations between the pad and rotor, not the pad itself. When the vibrations get into an audible range, you hear a squeal-like sound because of the rotation of the system. This vibration has many causes: you could have a (1) malfunctioning caliper, (2) warped rotor, (3) fatigued or missing brake hardware, (4) unlubricated metal-to-metal surfaces in caliper and brake hardware, (5) pads and rotor not burnished in, (6) malfunctioning brake booster, or a host of various and sundry brake system failures. The fact of the matter is that some brake systems are just born squealers. That is why most systems use brake-silencer shims.