2nd recommender choice

are you a recent grad? if so, do you have a professor or someone from your university who can write one for you? i was 2 years out, and i actually used 2 professors from my undergrad for LORs (one was a professor i had, and one was a professor i worked for).

if you can't do this (or if you're too far removed from undergrad), then i would say do the other supervisor. normally you'd want the person who knows you best to write your LORs. However, you also don't want a peer writing your LOR. it should generally be someone who is your superior (either a direct supervisor or indirect supervisor). there may be exceptions, but a coworker/teammate is not a good idea IMO.