Heya,
I'd get the 24 F2.8 IS if you want basically an L quality lens without the red stripe. The IS is very useful.
The ONLY benefit to the 24 STM is that its a pancake design. It's decently sharp, not fast to focus (we have one, it's sluggish like the 40 STM is and isn't silent), but it's great because it's so small. That's it though. You get the 24 pancake because its a pancake and you're wanting the smallest kit possible.
If having another 2 inches of lens on your camera isn't a big deal, the 24 F2.8 IS adds everything superior. You pay more. You get more though. The IS alone would be what I would suggest it for, along with silent, fast AF. But the 4 stop IS is really, really nice. It'll be nearly as big as the SL2, hah, but again, I would say its worth it.
So just figure out if you need smallest size possible via a pancake design, or if you want the best glass between the two and IS which is very useful, with basically L level quality. Price difference is there of course.
Very best,