It's all a matter of personal preference. I personally like the solid 8s because of the symmetry in the shape of the number. Whether you rotator the note; view it from the back against a light source (I believe eyevet coined the term "transluminescence note"); or flip the note about the x- or y-axis, you get the same pattern. Symmetry is what give 8s its rotator property. The only other number that can do this is 0, and solid 0s are only available on Specimen notes. Being a lucky number in Chinese culture is just an added bonus.
As Ottawa mentioned, 8s are lucky in Chinese culture because it rhymes with the word for prosperity/fortune/wealth. Does it really mean anything or make a difference? Probably not, it's just superstition, but 8 and 4 in Chinese culture are analogous to 7 and 13 in Western (think Lucky 777 and Friday the 13th).
Chalk one up for solid 8s.
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