I'm going with the Potenzas for a few reasons.
I know that tire pressure increases help rr with diminishing returns, and 51 is already pretty high.
Secondly, I think CapriRacer said that wider can be better for rr:
Barry's Tire Tech, rolling resistance part 2
I'm also assuming that the donut spare is built heavy-duty, and that you won't be going fast enough for the improved aero of the skinnier tire to matter much.
But now you got me curious.