On that bike, I'd say black walls. I love the look of the wide whites on deluxes or even heritage's that are two tones, but not so much on solids, especially black.
They really aren't hard to keep clean...especially on a Softail where you can get to the tires easily enough:
Wet tire...Spray on Westley's White Bleach...rinse off...use Mr. Clean Magic Erasor & spin wheels (assumingyou have a cycle jack)...rinse again and dry. It only takes about 15 minutes on my RKC and I do them once a month during riding season.
They are harder to clean if you ignore them all year and let them stay dirty.