M4/3 or APS-C

Try getting a faster lens before considering a potentially costly switch to a different format. If you're sticking to kit lenses, you aren't going to benefit a whole lot from a switch (and you'll lose portability).

Depending on what you're shooting 14/2.5, 20/1.7. 45/1.8 are all solid choices that won't break the bank and get you better low-light capabilities.

Flash can be a superior solution for all the reasons given by the previous posters, but from your description, there are going to be times where you won't be able to flash.