In absolutes, tip up gives a superior grip as the knife is drawn. The pad of the thumb is in contact with one side of the handle over the entire pad, while the opposite side of handle (clip side) is grasped sequencially by each finger until it is completely withdrawn. As soon as the pivot end of the knife clears the pocket (or waistband), the thumb can be used to open the blade.
That being said, linerlocks make a risky tip-up carry because the blade can open in the pocket and stab/slice your hand as you try to draw or dig around in your pockets for loose change, photon light, etc. I have several linerlocks and prefer that lock mechanism to lockbacks due to the ease of opening. All the linerlocks that I use can be flipped open with some effort with the factory pivot pin tightness.
Also, many times, my knives are not on my person, but clipped to a pack, vest, or strap where the tip-up position may not be the easiest method of draw.
Last, tip-up knives don't sit quite as deep in the pocket because of the taper of the typical blade neccessitates a taper in the handle, which will not support attachment of a clip (or enough strength for a molded clip a la previous generation Spyderco Delica/Endura)
Mostly though, it's personal preference.