BamZipPow's undertray thread should be useful to you:
http://ecomodder.com/forum/showthrea...pan-24777.html
Comparing mods alone, I would guess that an undertray works better than side skirts, but you can combine both of them for an even better effect. Side skirts are a lot easier to make than a whole undertray, that is part of the reason you see those on semis.