This time last year during 1L, I would probably have taken the firm. I didn't know what I wanted to do, and was hoping to get an SA gig. The experience in the firm will be helpful for OCI next year and you won't have to deal with questions like, "Oh, I see you worked in the criminal law division...do you want to be a DA/PD?" No matter how you answer these questions, all the partner interviewing you thinks is "Hmm. They just want to work for me to pay down their debt and then bail."
I ended up working for a D.A. however, and really loved it. It was something unexpected for me, and I had never really thought that criminal law would be something I would enjoy. Because you'll get good experience with the court, see trials and litigation, and assist writing opinions, that's all still great experience you can spin with a firm. Plus, if you're not really familiar with a D.A. or PD's job, then it's a good opportunity to observe some of that, and really know that that sort of career isn't for you.
As for me, when I took the D.A. job, I did it mostly because I didn't find any firm positions. I'm glad it worked out that way, but that's not for everyone.