Carried right front pocket:
-tip up allows the blade to be pushed against the side of your pocket to restrict it from opening in your pocket, although this has still happened to people using both methods of carry
-tip up moves the jimping and thumb stud towards the bottom of your pocket where they can't scrape your hand when you put it into your pocket
-tip up is needed if you want to use a "wave" for opening the blade
-tip down is faster to draw and deploy, and more secure, for knives significantly over 3" length, there is no advantage for shorter knives
Carried any other way, any other pocket, there are different pros and cons for each carry method.
I started carrying folders with pocket clips about 20 years ago. Back then almost all of them were tip down. The more expensive knives with G10 and liners tended to be tip down, the lower priced knives with molded handles tended to be tip up. Since then that has changed and there are probably more knives available these days in tip up. Of the 12 or so knives in my EDC rotation, they are about half tip down and half tip up, so I get plenty of experience carrying both ways to reinforce the above points.
People tend to pick one or the other, and generally they feel strongly about it. People that like tip down usually list the reasons why they like it. People that like tip up usually don't give reasons, or they like it because the blade supposedly won't open in your pocket.
I find it interesting that Spyderco makes the Para2 which can have its clip on either end, and they ship it tip down. Spyderco also makes the Bradley folder which can have its clip on either end, and they ship it tip up.