Both are correct. We use both of them. I think that "皆さん" is written much more often on a document and handout. I make several company's websites, and the expression ”皆さん” is suggested to use on those websites. It's a rule of those website guidelines. But it's so easy and speedy to write hiragana "みなさん" that I often use it when writing memos.

I think , everyone=minasan Ladies & Gentlemen=minasama minasama is more polite than minasan And we often say 'minasan oyasuminasai' or 'minasan konnnichiwa' usually we barely use minasama in addition, correct pronounce is 'ko nn ni chi wa' :) My English is poor so I'm sorry if you couldn't understand my sentence...

There is actually no difference. Both are correct, the only thing is that みなさん is the hiragana form and 皆さん is the kanji stroke form, which will most likely ALWAYS be present in any text you read in Japanese, so you gotta memorise most of them, such as 世界, which is the kanji stroke for せかい, 心, which is the kanji stroke for こころ, 俺、kanji stroke for おれ, etc in order to read a newspaper, for example... =) .. Good luck with your learning!!