I think that would be 「XとYの違いは何?」
it somehow sounds like you're quoting something... just 「XとYが違う」means "X and Y is wrong/different"(depends on context ofcourse) so in speech it would sounds something like "what is "X and Y different"?"(「X とY が違う」は何?), do you see where i am going?(yes you should use the quotation particle "と" but alot of people use は)
はい、久しぶりですね!
well, as you already wrote, 「って」 is also a Quotation particle, but that can also be used as a soft 「は」, like...「焼肉っておいしい?」。Using 「と」would be more formal. 「と」is just not a way of saying "and" and quote, it can be used to say "if" too. たとえば、「ここから落ちると死ぬよ!」"if you/me fall from here I'll/you'll die"(the pronoun depends on context, again :) )