Go for the smaller tube - its range of expansion will handle that tire size OK. The larger tube size will be easier to get kinked/wrinkled.
I carry 1 spare inner tube on my R65 - it is sized for the front tire, but will also work in the rear . IF I bought one sized for the rear it would actually bunch up/be oversized for the front.
With other airheads with different sized wheels, well, you gotta go with one of each, or, go for the statistical probability that more flats occur on rear wheels than on front wheels - and just carry a spare rear tube.