Ideally, for an event like this, if you can use flash, you're probably better off. If adding a lens is not an option, then the kit lens with a flash will give you the most flexibility. If flash is not an option, then you might want to try and use the 35. Of course, bringing more than one lens can help if you can swing it.
Remember, though, if you use flash, to diffuse it and get it as far from the lens as possible. That is, if you can use a flash bracket, that's good. If not, make sure you diffuse it somehow. I find that a sheet of bright white foam (which you can pick up in a craft store for less than $2, possibly less than $1) cut in a symmetrical trapezoid shape and rubber banded to the flash makes a very cheap, but fairly good diffuser. You use it by pointing the flash upward and letting the foam reflect the light toward your subject. You can still point the flash at a wall or a ceiling if one is handy.
Of course, part of what will govern your approach is what you feel comfortable with. If you don't mind looking like an old fashioned newspaper photographer, or having people look askance at your homemade diffuser (and possibly attracting a bit more attention to yourself), then you can get some pretty good shots this way (there's a reason why the old-fashioned newspaper photographer stereotype look exists), but if you're not comfortable with that, you might be better off making-do with a good low light performer like the 35mm. In any case, and especially if you decide to try and use a flash, I suggest that you practice in a venue with similar lighting conditions if possible. Don't be intimidated by manual flash settings. It's not as hard as you might think, and it gets the best results once you figure it out (I admit I'm a bit out of practice at the moment, though I am starting to brush up in anticipation of upcoming events).
P.S.: Just because the Sigma flash is P-TTL doesn't mean you have to use it that way. It almost certainly has manual settings and possibly an "Auto" setting as well. I think that's supposed to be a pretty good flash, so it's probably worth taking the time to learn how to use manually.
Last edited by CFWhitman; 07-09-2015 at 08:43 AM.