There's a lot left to the debate over disc brakes for road, but for mountain, "all road" or "gravel" or whatever the kids are calling it these days, and really cross (I saw zero people using cantis in last weekend's World Cup race), it's all disc all the time. There are some valid points of resistance to them for road (perhaps chief among those being the garage full of rim brake wheels a lot of you have) but one of the consistent complaints about discs is that they squeal like stuck pigs. Set them up right, and they're quieter than most rim brakes.
First step is parts prep, starting with scuffing/cleaning the pad faces. Use a Scotch-Brite pad like we do, or even some mid-grit (220 or so) sandpaper. This is especially helpful for used pads which may have oil or other schmung in them, which contributes to noise and detracts from slowing you down.
After you're done scrubbing the pad surface, clean the whole pad with rubbing alcohol.
Next, we're going to grease the pads. WHAT?!?!?!?!?!? Grease the pads?!?!?!? Yup - except you're going to grease the BACK side of the pads, comme ça:
We use red boat trailer bearing grease, but any thick grease works. The critical thing is that it's thich enough to stay put, and the more waterproof, the better. Disc brake silencer from the car parts store works just as well. What the grease is doing is muting the resonance that occurs between the caliper piston and the back of the pad. It works like a champ. Be very careful not to get ANY grease on the pad face.
After that, very carefully reinstall the pads in the calipers, and clean up any errant grease that may have gotten onto the caliper body.
Next, scrub your rotor. New or old, do this. Very important not to get any skin contact with the rotors. Give them a good hard scrub, to the point where abrasions are visible on the rotor surface.
Even though this rotor got a few small rust spots after a super wet ride, it's silent thanks to the good scrubbing. After the scrub, wash the rotor off with your rubbing alcohol.
Next you reinstall the rotor on the wheel, then install the wheel in the bike. Following the instructions for your brakes, align your calipers with the rotors, making sure that they are as square to each other as possible. Misalignment makes noise. Torque everything down to spec, as movement also creates noise.
Now it's time to bed the rotors and pads. Get going about 10 or 12 mph, and then make a medium hard stop. Repeat that a few times, then get going faster and faster and make harder and harder stops. This whole bedding process should take maybe three or four minutes.
And that's it, you're ready to go. When it's really damp out, your brakes will make a bit of a groan at the first stop (we get lots of foggy mornings in Newport to test this), and they'll be loud when they're downright wet, but your rims brakes probably do then, too. And unlike your rim brakes, your disc brakes will actually work really well in the wet.
This prep will usually last the life of your pads, unless you ride in some really excessively nasty conditions. We prefer sintered pads to organic. Each has its strengths and weaknesses, but to us sintered works in all conditions and ultimately has more power. Which you prefer is up to you.
I have absolutely no tolerance for squealing brakes, to the point where I can promise you that there's no way I'd ride a disc road bike if loud brakes were an unavoidable part of the equation. They simply are not.