The RR RB
-211 series offered on the B-757s do have a much lower maintenance cost and can stay on the wing much longer than the PW
engines can. The RRs also have a higher thrust rating.
2037 and 2040 series offered on the B-757 cost less and have a lower SFC than the RRs. But they also offer less thrust and higher maintenance costs.
Now LCCs that have operated these engines on B-757 models include ATA (RR RB
-211s on both the -200 and -300), Song (PW 2040 on the -200), and National (RR RB
-211s on the -200). There may be others, but those are the first LCC operators of B-757 models that crossed my mind.
I might add that UPS operates B-757-200PFs with both engines. IIRC the first 20 or so B-757s they bought are equipped with PW
-2042s, and the last 20-25 airplanes they bought came with RR RB