QUESTION 1: Does it matter which stance I use?

ANSWER 1: No. It really is a preference thing. You will see both used at the highest levels and everything in between. The main concept to understand is the "foot up" promotes higher vertical jump (allows for higher contact) and "foot back" promotes greater horizontal movement towards the net of which both increase one's window of  acceptance over the net. I personally use the "foot back" stance but teach both based upon players natural preference. Nothing wrong with experimenting with both to find out your preference :-).