My advice would be to go for the mid size if you can. I dont know what will give you the best shot in OCI, but I have friends that did in house jobs there first summer, and the places didnt do offers. So, if 3L OCI falls through, you are going to want to have something to fall back on, and the mid size firm is going to be more likely to wanna keep you around.Firms are still giving out interviews and stuff, so dont give up yet on big law this year.
I still have a little time to decide, but I'm leaning towards the in-house option. (It's actually Fortune 100--not that it makes big difference.) The main thing is that I'm really not interested in the small-medium firm, and I can just as easily get the same type of offer again next year from similar place. I know that the GC does not hire straight from law school (thank you all for assuming I know nothing...). What I am really most concerned with is what looks best on my resume for 3L OCI, i.e. the prestige of Fortune 100 versus the practical experience of a law firm. Also, thanks to those who suggested big law in small markets, but I am unfortunately tied to the city where I live now.