As someone who pledged as a junior (and befriended all of the juniors and sophomores in my PC), my opinion is that it doesn't matter. A senior, junior, or even sophomore--as long as the girl is someone you click with--will all make a great big for you. Almost all of my close friends in my PC have bigs who are either the same year as them, or even a year younger. It works out fine!
Bottom line: Choose someone you LIKE and who is INVOLVED in your chapter... someone who can truly mentor you and help you get to know other sisters and get involved in the chapter!
Don't worry too much about their year in school.