I'm seeing the Shimano Nexus/Alfine 8 listed as:

Ratio 0.527 0.644 0.748 0.851 1.000 1.223 1.419 1.615

https://www.sheldonbrown.com/nexus8.shtml

So, on the top end, you have just over 1.6x gearing, and on the low end, 0.5x

If you go with, say 42/20, then on the low end, you get pretty close to 1:1 gearing which is quite low. On the high end, you get about 3:1 gearing which is moderate.

Do you have a standard geared bike you can compare to? What are the crankset and cassette/freewheel ranges?

If it was me, I'd bias it slightly towards higher gearing, so I'd probably choose the 45/18, but it really depends on what you're comfortable pedalling..