It sounds like you like what you're doing now at the metro, and that you've already all but made up your mind.
But if you do take the metro part-time job, go in with your eyes open. Make sure you realize that, although you will be working at the paper, you will not be working for the paper. You will be working for the temp agency. What you will be in, essentially, is a slightly-more-secure-and-regularly-timed, but-still-a-freelance, or-contracted, position.
This can be a good thing these days. You will not actually be on the paper's budget. Ergo, you will probably not be a target for a layoff, because when the bean-counters start counting, they won't start with the beans that you're among. They'll be considered separate, different piles of beans. But be cognizant of your place going in.
You need to realize -- and this is a biggie connected to what I wrote previously, and easier said than done -- but you need to understand that you are not truly part of the staff. They are making no real, official commitment to you. This was always a sore spot for me -- one of my own doing -- during times I freelanced for writing, or the one time I had a temp job at a paper. I did it first to break in, and then when I'd been out of work a few months and needed a job, and I was always treated well. But..part of what I love about newspapers is the being a part of it; and I never truly felt like I was unless/until I was officially hired.
But if you're fine with the set-up, and know that you'll always be fine with it (because, face it, it may never change; things are bad at papers, and really, why should they hire you if you're doing the work anyway?), then this could be a great opportunity. Especially if it's the type of work you want to do for the long-term, anyway.
If that's the case, there are two other things to make sure and do -- preferably before you accept the metro job. First, check with both the temp company (the agency) and the host company (the paper) and make sure of and be clear about just how temporary this job is. (Is it long-term, but seasonal? Only for the duration of the school year? Is it anticipated to start over again each September? If so, will you likely get a call-back? Or, is the job truly indefinite, or what? How/when will I be paid? What is covered, and expensed, etc, if anything, and how?).
It sounds like you're trying to do some of this, but be sure to dot the i's and cross the t's.
Then, once you address the particulars, try to feel out the newspaper about the future, and if/when a staff position of a similar level/responsibility might ever be available. Try to get a sense of whether or not you would be considered among the top candidates for such a spot.
Major metros often go outside for things, trying to get the very best. That is changing somewhat because of budget issues and the need for papers to just go with what they've got.
But that brings up another possible issue: Would the paper fill a sports desk opening simply by moving someone -- a staff someone -- with probably more experience and seniority than you who needs a safe place in these turbulent times -- from in-house (maybe even from another department) rather than hiring you, or anybody else, into it?
I'm probably bringing up too many possible scenarios here, but the point is, it behooves you to try to find out just how much they like you and might be willing to commit to you, even if you might have to wait awhile for it. You should sort of do an interview of them.
Of course, you could think you'd be OK, and fine and happy with everything at the metro regardless -- I did that once -- and just decide to deal with the future when it happens, too.
I just wanted to put some things out there for your consideration.