There is no significant difference. Just keep your string tension balanced and whips and slacks should be fine. I will say that heavier strings are usually easier to whip.
THere is no real difference in string as long as String Tension is good.
Most people say that heavier string (Mostly Cotton) is best for those type of string.
I have used 100% Poly, 50/50, and Cotton and I personally don’t feel a big difference.
If you do it right, yes, it will open enough for you to catch it. There are also small moves that yu can do to make the loop bigger. Andre shows you one where you go over your NTHF like the second roll in MAtrix. I think adding momentum will definitely make the loop a little bigger.
I think 50/50 cotton polyester strings would be the best for whips and slacks but just remember to keep your string tension right.(other string work fine too but I just think 50/50 cotton/polyester slick6 and 8 strings are the best.
Alchemy strings are by far the best I’ve used for whips and slacks. I believe they’re formulated with a nylon/poly blend so they feel heavy and assertive during whips and lacerations. PM me for a link.