I love both, but I think it mostly depends on what type of ornaments you have. If you go with eclectic, colorful, or playful ornaments, the white lights will highlight them (colored lights might make them look too chaotic or cheapen them). If however, you do more traditional ornaments (think red balls, gold bells, etc.) or a limited color theme (white and gold, all red, etc.) then the colored lights will definitely add to those.
OR...just mix it all up and have fun!! It's a Christmas tree, and will be uniquely yours either way.