Hi! They both mean and are more or less pronounced the same :) It is just one is used commonly in North American English spelling (words like recogniZe and realiZe) while the other is used commonly in British English spelling (words like recogniSe, realiSe). So it really depends on where you are and which spelling is commonly used in the region. Hope this helps!

Also, when referring to two separate things you would use the term "these" instead of "this". Great question little one;] Not everyone picks up on such subtleties.