Either should be fine.
BTW, my friend's grandfather gave me advice on lines for slab reefing a long time ago... he said the reefing lines should be one size smaller or the same size than the smallest line you'd use for a halyard
... given that the lines are all holding the same sail in position, I think it makes sense to size them that way.
Yeah, I'm not certain of my halyard size, but I believe they're 7/16", so 3/8" for reefing lines would probably make sense. I mean, my outhaul is like 5/16" (if I remember correctly), so 3/8 should be OK for reefing. I know the cleats on the side of the boom weren't made for 7/16"--they're really not big enough. Thanks again, SD...