There are few elements of a guitar build less sexy than selecting pots and capacitors. You don't see them, you perhaps don't relish the prospect of installing them, but they are a key element for making your guitar sound and play how you like. Thankfully they are actually fairly simple and once you know how they function, you can make educated choices about what you want on your own guitar rather than just going for the 'standard' arrangement.
Lets take a look at pots first...
There are two basic types of volume and tone pots, linear or audio taper pots. To describe the difference I'm going to go out on a limb here and assume most guitarists are like me in that they do not use the volume control as a volume control but rather as a gain control. That is to say that I set the gain on my amp to the maximum I want, with my guitar volume maxed out. I then use the volume control on the guitar to dial out some of the gain by rolling it back for rhythm parts and then roll the gain in again (by increasing the guitar volume) for lead parts. The way the gain is rolled off and on is based on the type of pots I have installed.
A linear pot will reduce the gain, bit by bit, gradually and evenly until all the gain is rolled off, because it has a linear response to you turning the knob. But the problem with this is that I only ever want to dial out about 20% (max) of my gain which means I'll only ever turn the knob about 20% of how far it can actually be turned and I'll need to find my 'just right' tone within that 20% of travel (of the knob); which can be tricky.
This is where audio taper pots come in handy. Audio taper pots, by design, dial out a small amount of gain initially and then once you're at about 50% (estimate) of the travel of the knob they start to dial out gain quicker, meaning that you effectively have a much larger 'window' for sculpting your tone and gain. So it means that less gain gets rolled off or on for the same amount of turn of the knob, giving you more control and more chance of getting just the right amount of gain. The same principal applies for the tone. So who is this useful for? Well its useful for anyone that plays driven guitar but particularly for those that tend to play relatively lightly driven or edge of breakup tones because its a very intuitive way of getting just the right amount of dirt in your sound.
So what does 250k and 500k refer to and why are 250k normally used for single coil pickups?
Okay, I'll get a little technical here to explain what's going on. If you imagine a tone or volume pot is just a dial in between the guitar pickup and the guitar amplifier. The dial can be turned to increase of decrease the amount of resistance to the signal to change how it sounds. So the 250k and 500k refer to ohms. The rule of thumb is that the lower the number, the more high frequencies will be cut from the signal therefore (at the very start) a 250k pot will cut more higher frequencies than a 500k pot. This also tends to be the reason that single coils tend to be paired with 250k pots and humbuckers tend to have 500k or higher pots.
Generally speaking single coils tend to sound more pleasant with some of the shrill or high end dialled down a bit, whereas humbuckers are naturally rounder and warmer sounding already and therefore need less 'taming'. These are simplified generalisations but you get the idea.
So what about capacitors?
A capacitor simply removes some frequencies from your guitar signal. As the guitar signal passes through the capacitor it filters out some of the frequencies so that they are
not reflected in the sound that comes out of your amplifier. The lower the value or number of the capacitor, the brighter your guitar will sound, and the higher the number,
the warmer and more bass heavy the guitar will sound.
So doesn't this just mean that if you've got a telecaster you just use a .015 (low value) capacitor because Tele's are supposed to sound bright and twangy? Well no, because you need to find the correct balance of brightness and warmth according to your tastes and the types of pickups and pots you've selected. And bear in mind that capacitors work in conjunction with the pots to shape how they perform their task. Think of your pots and capacitor as providing you with your full range of tones that you'd like from your guitar, not just one sound.
The good news is that capacitors are only a few euros each so a good idea may be to buy the most common value capacitor for your setup and then one or two others on either side of that value to test, you'd be surprised by the difference, its not one of those things that only trained ears will hear, and you will certainly have a preference.
So what about materials and quality?
Pots and capacitors are very low cost components so there's really no need to save money here. A $15 difference per guitar to use quality components may be a big cost for a company producing tens of thousands of guitars a year but for us small builders, its peanuts. Get the best you can because they make a difference. As for material (in capacitors) there are theoretically differences in the way they sound. I cannot tell a difference, I don't know anyone who can. The important thing is the value of the capacitor not the material, in terms of what you will hear out of the amp.
So what do you use in your guitars and why?
Everything I use in my guitars is what I consider to the very best Telecaster hardware (according to my taste). I like a guitar tone that has warmth and sparkle that is similar to a strat on the neck and classic Tele twang on the bridge with heaps of punch (i.e not too brittle sounding). I also use my volume as a gain control. So I use Fender Custom Shop Twisted Tele Pickups, CTS 250k audio taper pots and 0.047 orange drop capacitors.
So will that work for you? Who knows, but most importantly, don't listen to anyone that tells you there's just one setup to use for your guitar! Experiment and let your ears
and heart decide on what you like.