I don't know the other brands. The prices are so close, I would just say go with Vishay, it's well known and respected.

why do I need to calculate it? or I dont? as I said it goes to an op amp. no needie to calculate anything

This is arrogant thinking. You should always do calculations to determine if the value you picked is right for your system.

As for which value to use, I would pick a higher resistance, rather than a lower one, since you're buffering it with an op amp then you don't need a low impedance source. The higher resistance will give you smoother control because you will have more resistance per each degree of rotation. For example, a 10 turn pot has 3600 degrees of rotation, so a 100 ohm pot has 100/3600 ohms per degree, or 27.8 millohms per degree. That's so small it's going to be hard to control and going to be jumpy. On the other hand, a 20k pot has 5.6 ohms per degree, which you can control more precisely and get a smoother response from your pot.

You want to go high, but not too high, especially for a reference voltage, as the higher resistances have more Johnson noise (thermal noise). For example, going up to 100k might seem like a good idea because it wastes very little power, but it could actually be bad for your system if it introduces noise that affects your system. I would aim to keep the resistance as high as you can go (to minimize wasted power and get a better control) but to still keep the thermal noise below 1 LSB of your system. So determine what 1 LSB is for your system and then use that value and the thermal noise formula to determine your maximum resistance you can tolerate.

Where:

*k* = boltzmann's constant (1.374 x 10

^{-23} J/deg K

T = Absolute temperature (deg K)

R = Resistance in Ohms

B = System Bandwidth (in Hz)

You want to keep E

_{N} below 1 LSB. Also remember that your temperature inside the box might be around 40 to 50 C , or 313 to 323K.

as an example: a 16-bit system in 50kHz BW can easily use a 20k pot, but a 24-bit system would need to use no more than a 100 ohm pot.

There are many reasons why you need to calculate it. I've shown you at least two of them here.