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!!