Is it better to resurface old rotors and use new brake pads, or just buy a whole new set of rotors and pads together?

 

You can do either with no issues. Unless the rotors are damaged a seasoned resurfaced rotor is slightly superior to a brand new rotor because it has been heat cycled extensively and is less likely to warp during break in.

 

...unless they use the wrong type of lathe in which case stopping distance is increased along with noise and chatter.



Unless rotors are warped, deeply scored or otherwise damaged, a "pad slap" is usually going to be fine.