Difference between Sicilian and Neopolitan pizza

I don't think there's a uniform Sicilian pizza (there is a bit of variation depending where you are in Sicily) but Neopolitan pizza is quite recognizable and follows certain guidelines. In short, the latter is a bit "saggy" and uses a certain set of ingredients described below:
https://americas.pizzanapoletana.org/foto/allegati/AVPN_Disciplinare.pdf
https://en.wikipedia.org/wiki/Sicilian_pizza

I wouldn't compare either one to their American versions, although there are places that make authentic Neopolitan pizza in the US and they're certified as "DOC" ("a designation that made AVPN a legal entity able to give special designation to pizzerias who meet strict requirements that respect the tradition of the art of Neapolitan pizza making")

PS. This is a great article about the variety of pizzas in Southern Italy:
https://www.bloomberg.com/news/features/2018-03-15/the-wild-pizzas-of-southern-italy-have-to-be-seen-to-be-believed

Neapolitan pizza crust is quite thin. There is a version of Sicilian pizza called "sfincione". It has a very thick, focaccia-like crust.
http://www.bestofsicily.com/mag/art250.htm

Enjoy them both!