6/6 plus or wait for 6s ??

I'm currently using an Iphone 4 which is super old and couldn't update newer versions. It often get lagged and my mom is willing to get me a new phone which I'm super thankful for. She said if she get me this phone I would be using it till college or till I get a job to afford myself another one if I want to change, means I only have one shot. I'm currently a sophomore in highschool and I was wondering whether or not I should get the iPhone 6 or 6 Plus since I heard a lot of pros about it. Or should I wait for the 6s (if it were to come out) ?? Which phone would be better for me for long term use? I'm a typical teenager and I use my phone all the time. PLEASE HELPPP!!