I really don't think that either is better than the other when seated. They are both difficult to get to when sitting. If you want something that is great for seated drawing, you really need to look to a shoulder rig or a chest rig (more tactical). Thisgh rigs also stink when drawing when drawing from a seated position... especially a car, where thigh room is a hot commodity when wearing gear.
The only reason you see more retention on low-rise holsters is because they tend to be used more often for duty use. Almost all holsters that I know that come in an option for low or high ride have the same level of retention, only vatrying the hight of the holster ride. Most guys that I know carry level 2 high-ride holsters for duty... then again, we are all plain clothed.
...which brings me to the answer of the biggest difference (in my opinion) between the 2 carry techniques. High ride holsters offer better concealment, but make for a harder draw. It is harder to pull the gun nearly up to your armpit to get it to break leather.... like a desantis I used to have for my Glock. It makes for GREAT concealment, but comes at the cost of speed.
Lower is most often used by uniformed personnel because concealment is not an issue and it proves to be a little faster... and more easily accessible wearing jackets and such.
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