I think this will boil down to preference already... in anycase, haha I know my expression was kind of over-reactive, but it was just an expression.
Both are equally good lenses, but given the prices, I'd go for the F2.8.
With Flash, there are 2 ways to see it. If you have flash, most probably you're at least clearing 1/60s and you probably can push your shutter up and pump your flash harder for hand-holdable speeds.
For me, I try to avoid relying on flash for main light, to me, flash is strictly for filling or at best abit of lighting compensation. (This would be in events context, portraits would be a different story). If shallow DOF is not an issue, I'd still shoot at F2.8 even without flash, to catch as much natural light as I can. Even if that means pushing ISO up to 800.
Then again, I'm not pro with the flash, so others may feel otherwise. But all in all, I prefer speed over the IS. The bit of range can be compensated I guess, although the range makes a good walk around for FF.
In discussion with some pros, they expressed that if you do events, most of the times you won't need faster than F4 since you will want to capture everyone in your pics and need a big DOF. That may be the best reason I can think of for choosing the 24-105L instead.