Cushie Tushies are just a one size, all in one nappy. Ittis, you can buy the Itti Bitti D'lish which is sized (small, med, large), there is an all in one or a 'snap in one' (all in 2) version, they also do an Itti Bitti Boo which is a fitted nappy (needs a waterproof cover), these are also sized. They also now do a one size version called the Tutto.
Ittis are usually plain (although they do bring out limited edition prints occasionally), whereas Cushies are prints.
I personally found that Cushie Tushies were a bit more absorbent than Itti (the D'lish, anyway) but other people may have found different.