Just personal preference. It's more natural for me in the back pocket, both for carrying and for withdrawing quickly.
That being said, I use both front and rear pocket holsters. If I'm going to be mostly standing and walking, always rear pocket carry. If I'm going to be sitting/driving, I may switch to the front pocket.
My front pocket holsters still work just fine in the rear pocket, so it's easy to switch back and forth throughout the day, depending on what I'm doing.
If I had to pick just one, I'd say........

I'm really not sure.
I'm glad I have both!