Which joystick do you have? Say for instance you have a sanwa jlf, then changing the restrictor is very simple. You simply pop it off and put the new one on, no extra tools needed. It only takes maybe 2 minutes in total.
As far as which one to get different people will say different things. I use square restrictors on a couple joysticks and I use octagon restrictors on other joysticks. My personal preference for fighting games is the octagon restrictor, others will swear by square, and the rest will swear by circle. It's really one of those things where you just have to buy them and try each one out until you decide which one feels best for you.