Personally, no much of a preference. However, it really depends on the body type of the lady.
if the lady is:
slim, not busty : then either no tan, or full body tan. Tan lines tend to make small breasts look flatter. If that's the intended effect, then by all means.
medium or plus build, and busty: go for the tan lines. very turn on to look down and see a nice tan bottom with white g-sting tan lines.
A good tan also makes a full figure body looks slimmer.
I find that tan doesn't go well with red hair ( both natural or not ). Pale natural skin colour is the best for red heads.