Like (I think) Greg_L, I don't really understand what the OP is asking.
The guitar (or bass) pickup affects the sound of the guitar (or bass). Depending on what you're changing, the effect could be so minor you don't notice it, or huge. It affects the sound of the instrument when you're recording pretty much the same as it affects the sound when you're playing live. It affects "the mix" (live or recorded) in that the sound of that particular instrument changes.
I'm
guessing the OP is asking about switching between the bridge and neck pickup. On most electric guitars, that makes a pretty noticeable difference in the sound, which should be fairly obvious. It would typically be much more significant than switching between an SM57 and an SM57 knockoff, though that - of course - depends on just how crappy
the SM57 knockoff is.
Other possibilities, among many, are that the OP:
- Is thinking about changing out the pickups in a guitar. Pretty much the same answer, though obviously it depends on what you're switching for what. Replacing a Strat (say) pickup with a pretty much identical replacement won't make a lot of difference (perhaps like replacing an SM57 with an SM57 knockoff). Replacing it with a high-output humbucker will make a lot more difference.
- Is talking about an acoustic guitar pickup. This might kind of make sense. It's very common to use a pickup (piezo or whatever) when playing acoustic guitar live, because micing the guitar poses some challenges in terms of feedback and maintaining mic position. It's more common to mic an acoustic for recording, though you could use a pickup (or both, for that matter). The choice will affect the sound of the guitar in a fairly easy-to-perceive way, which will (of course) depend on the pickup and how you're micing it.