tip up for me, i dont think it really matters as long as ya get one style, practice with it, and stick with it. since a high % of my practice has been with tip up it is what works best for me.
theoretically tip down would seem to have less chance of opening in your pocket. in real life the only 2 that have ever opened in mine were tip down (spyderco khalsa and gunting) go figure. i carry a lot of tip ups too (ie emerson specwar '8, spyderco Ti ATR, strider SnG& SmF, ER fulcrum, etc) & have never had a problem with them knock on wood.
i have been told ya can draw tip up faster but i dont know about that, maybe?