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myprshe, I'm new to the forum, but not to knives in general.
The advantage that many find with tip up carry (pocket clip near the rear of the knife) is that when you reach in your pocket to extract and deploy your blade, your hand will already be in pretty much the same position it needs to be in to open the knife via the thumbstud/thumbhole/flipper/button etc. All you really have to do, depending on the size of the knife is scoot your hand down a bit to reach the deployment system.
With tip down carry (pocket clip mounted near the pivot), after you reach in to your pocket to grasp the knife, you need to pull it out and pivot it in your hand to be able to open it via the deployment method, generally. Ways to get around this are of course the aforementioned Spydie-drop or a brisk flick of the wrist, depending on which knife you have and how loose your pivot screw is.
While I admit that I do prefer tip-up for the reason I mentioned...the extra half second or less that it takes for me to pivot the knife in my hand is really a moot point, and not a deal breaker for me when I make a purchase. If a knife has the option for tip up carry, I'll take it, of course, but I own several knives with a tip down only carry option that I like just fine.
I actually have a Byrd Cara Cara 2 stainless that I have oriented for left-hand, tip down carry for when I wear a tuxedo for concert performances and whatnot. It rides in my left side interior breast pocket in my jacket. When I reach in to grab it, it comes out of the pocket ready to deploy (thus forgoing the usual pivot motion necessary with tip down carry). Similar to how you would change the cant of a pistol holster depending on where it is placed on your body, I have changed the pocket clip of my knife, if that makes any sense.
In the end, it comes down to personal preference, and how you plan on using your knife. Also, many knife brands that offer various mounting points (Kershaw is often guilty of this, I have found) for the pocket clips will tap the holes such that when the knife is tip down, the knife rides low in the pocket, and when it is oriented for tip up, there is a great deal of handle sticking up out of the pocket. Again...personal preference. I don't usually care about concealing my knives so much as I do not having them get caught on something and come out of the pocket.
Here is the first video in a quick 4 part series on this issue by Cutlerylover on Youtube. I'm sure you can find your way to the others.
http://www.youtube.com/watch?v=eJdE8aLW5yk