尤其means pretty very, an adverb used to describe degree of something. This is (pretty important.) 这件事(尤其重要) (This part is necessary for a sentence，we can't go without it) 尤其是 espicially , plus something as a specific example used to support our point. (this part is not necessary for a sentence) 他们都很好，(尤其是习先生。) They are all very nice, (especially Mr. Xi.)

I’d be interested to see other responses but for now: Far as I know, they mean the same thing. if you would need a 是 without the 尤其, you should say 尤其是。if you don’t need a 是，for instance when 尤其 is followed directly by a verb or adjective, you can leave it as it is. 那尤其重要 尤其是她 You need a 是 before nouns and pronouns, you don’t need it before an adjective like 重要，and that determines which of the two adverb forms you use.

尤其是+n. 尤其+adj.