First I should qualify myself: I am a current applicant just like yourselves so I do not have any "insider information" to share:
This is going to play out a number of times for me on the trail this fall too. The dinners are great ways to meet residents and other applicants in a low-stress environment. It will demonstrate interest and you can bring it up in interviews but ultimately I have had residents tell me, "Don't forfeit an interview for a dinner, the extra interview will increase your odds of matching (somewhere) over a dinner." I had residents at two separate programs pretty much tell me this verbatim.
All of this being said if a program is your pre-interview trail #1 or #2 I would do everything I could to make the dinner. In other words, if you are REALLY REALLY interested applicant you'd find a way to make the dinner work.
Hope this helps!