Upload & Sell: On
I like primes and zooms. I use both, as needed. It is difficult to go wide on crop bodies without zooms, so they excel for travel and events. You can't always step back far enough, like in the Coliseum in Rome.
For low light, I like fast primes, like the Canon 35F2 and 50F1.4. I use the 35F2 much more often than any of my other lenses.
If you are going to use bounce flash indoors, just stay with the 18-55 IS. It will do fine at F7.1 or F8. I have more expensive zoom lenses, but the kit lens is really pretty good, once you can stop down enough. It isn't as sharp as the 24-105L or other L zooms, but it also costs much less. If money is an issue, get a good inexpensive prime, like the Canon 35F2. If you want to spend more, go right ahead. However, you can get great images on a crop with this lens. If you get one that isn't sharp, send it back for a replacement. Mine has been great on 3 different crop bodies.
Great advice. I've pretty much decided to buy a flash as it will improve my indoor pictures no matter what lens I use. I still just can't decide if I want to go with a fast prime for indoors and a longer zoom for outside, or a Tamron 17-50 or Canon 15-85 for everything. I hate making decisions on lenses My budget is around $620 right now and possibly more if I sell the 18-55 and 50 1.8.
Sell them both, then you can afford the 15-85 and the Sigma 30