i was just wondering if you have taught 7th graders before? it can be really difficult and one needs to be prepared to deal with a lot of behavioral issues. from my experience, teaching younger kids is a lot more about keeping control of the classroom than teaching about biology or anything else. for what ever reason, which there are many, it is really hard to get little 11 to 12 year olds to focus . . . and behave. when the kids are good, it can be pretty cool, but when they are bad, they can give you grey hairs.
anyhow, if you haven't already had some teaching experience, i would make sure you get some before commiting a whole year to it. it isn't an easy job and it takes a lot of dedication.
i personally think that it comes down to what you would like to do more, both will offer unique opportunities and experiences and will look fine on an application.
best of luck.