Better to go with a longer spindle length bottom bracket or shorter?
I have just rebuilt a 1990 Raleigh bike (exact model unknown) and tried several bottom brackets to find the best. This turned out to be the one that brings the smallest chainring (in your case only chainring) closest to the chainstay without touching it. The one I've ended up with is 117.5mm whereas the original was 122.5mm. The front derailleur works perfectly (not applicable for your setup) and the chainline looks great, albeit aimed at the larger cassette ratios rather than the smaller ones.
My logic is that when on the larger sprockets I am putting a lot of pressure on the pedals, so a straight chainline is best for chain strength and wear. When in the smaller sprockets I am usually on the flat or downhill, so a bit of chain angle is not such an issue.
Thus, I would go for the shortest BB you can get away with to keep the chainline close to the chainstay, and minimise flexing forces on the BB stubs (are they called that?)