Heh, way to start a religious war Bob! We haven't had this particular battle in a while
In my opinion...
Tip-down's sole advantage is that if it opens slightly in your pocket, you won't cut yourself drawing it. The biggest disadvantage is that it is not possible to draw tip-up without using a pinch-grip at some point.
Tip-up, you can have you entire hand on the knife at all times, no pinch-gripping ever required. It's a more secure opening. It also means the heaviest part of the knife is in your pocket instead of above your pocket (or just at the rim), so it's less likely to work itself up and over.
Overall, I'm a much bigger fan of tip up, especially if the locking mechanism holds the blade in the handle well (has to be better than the ball-detent system).
However, tip-up or tip-down is not one of the things that will make the difference between me buying the knife or not. What WILL keep me from buying a knife is if the clip is mounted too low. Tip-up or tip down, MOUNT THE CLIP HIGH. If too much knife sticks out from the pocket, it scratches things as I go by, it's way more likely to work itself up and out, and people notice it more -- and most importantly, I know from experience I'll carry it less often.
High-mounted clip is the #1 priority. that does not interfere with the ergonomics of the knife.