I have on hand 3 cranksets that aren't being used.

1 is a 53/39 octalink
1 is a 50/34 octalink
1 is a 53/39 hollowtech (outboard bb)

My issue is that I want to be able to ride this bike as a backup road bike, in addition to being used as a cross bike.

For this reason, I wouldn't want to have a single chainring since it'd take time to swap out rings and/or cranks.

The 50/34 makes sense since I'll be able to swap into road mode quite easily, but that's a pretty huge gap and a 50 is pretty big for off road stuff.

I think I'm just going to go with a 47/39 so I can put the hollowtech cranks to use and see what that gets me.

The big issue is that I run an old octalink setup on my road bike (I can't change the crank), and I like to keep octalink parts and cranks around as spares for when my power meter needs to be taken off. I have the compact for VERY hilly races where the easier gearing makes up for the lack of a power meter.

The 53/39 octalink is a backup, albeit a strange one since the crank arms are 2.5mm shorter. I figure i can make that adjustment as needed.

I guess I'll be giving the 47/39 a go.