I missed this, as I haven't been on much the last few days. And because I am NOT a "senior member." I've not been on TPF nearly long enough, and I'm not old enough--don't let the early onset dementia, the middle-age spread, the deteriorating vision, the bad knees or the loss of hearing fool you.

But I absolutely agree with what seems to be majority opinion: Honor the commitment you've already made. Because, in the end, your integrity is worth FAR more than either of these jobs and it's not worth going back on your word just for the "hope" that this will get things going for you in Chicago.
Like SteveD, I wonder if there is ANY possibility of doing both? Tell the restaurant that you are SO sorry, but you've booked this specific amount of time, and then see if they'd like for you to be available later in the day, that evening, the next day, whatever. Yeah, you'd have a lot of drive time, but it's an option.
Plus, it would say to the restaurant that you are willing to do just about ANYthing EXCEPT not honoring another commitment--and it tells them that you are busy and "in demand" as a photographer, and that booking further ahead (especially for a grand opening) is a REALLY good idea.
Lastly--what I would NOT do is talk to the Senior about this situation and "explain" your dilemma to her. This is NOT her problem; she booked a photographer already. This is YOUR problem--back out of your commitment to her, or don't, but don't try to make it her problem. I think talking to her about it would put her in a VERY uncomfortable position. I mean, really, what's she going to say?? At 18, there's NO way I had the kind of confrontational skills to handle this. I'd have just said, "well, sure, I understand. You have to do what's best for your business." But I would have FELT like you'd backed out on me, and I would very, VERY likely not have hired you again. Heck, that would probably still be my response to someone TODAY, because you know, if they came to me and basically say they want MY permission to back out of a commitment to me, well, I don't think I want to rely on that person much anyway.