I'm an arranger, and I would have voted for Once over Nice Work.
Nice Work is extremely good, but it doesn't sound any better or different than any other 20s/30s show...one could argue that the orchestrations aren't even as good as "Crazy For You", and "My One And Only", the two other Gershwin Broadway musicals.
Once, on the other hand, had to create a world of sound that had never been heard on Broadway before, and do so with musicians who were also actors in the show. I wasn't around for this process, but I'm sure it involved more than handing out lead sheets and yelling, "go!"
I agree that it's silly to compare the two...the techniques involved are completely different.
Behind the fake tinsel of Broadway is real tinsel.