there is no difference. It's just another way of saying the same thing... You could say that すいません is a "fast" way of speaking すみません

There is no difference in meanings. In a written language, it should always be written as "すみません" but the way it's pronounced is sometimes closer to "すいません".

”すみません” is formal lines of "すまない." ”すいません” is slang word of "すみません." In the most cases, people say "すみません" by ordinary.

It's a little informal "すいません" than "すみません".