To put it in general terms that will be more useful to more readers for a longer period of time:

You're trying to decide between a newer lower tier camera and an older camera from one tier higher up. Which works best for you will still be based in which features are most applicable to the types of photography for which you plan to use the camera. In terms of low light performance, the most important thing is the sensor. How the AF system works in low light will also be a consideration.

• Both cameras appear to use the same imaging sensor. If it is not the same one, they are near identical in terms of both specs and measured performance. Draw
• The newer camera has a newer processing chip. There are cameras more advanced than these two that use each of these same chips. It's probably pretty safe to say that neither of these two cameras are utilizing all that each respective chip offers. Since you are planning on saving raw image data, the only difference in image processing will be how the JPEG preview (what you see on the back of the camera when you review the image) will look. It shouldn't affect the raw data collected by what appears to be the same sensor in both cameras. It might affect how fast that data is processed and sent to the memory card, but those other potential differences between the two processors are already reflected in things such as frame rate, buffer depth, shutter lag, etc. Draw
• The older camera has a far more sophisticated OVF (optical viewfinder) based AF system capable of operating in significantly lower light. 45 vs. 9 total AF points. 45 vs. 1 cross-type AF points. 27 (9 of which continue to operate as cross-type) vs. 0 AF points capable of operating with f/8 lenses or lens/TC combos. Big Advantage: Rebel T7i
• Both cameras have Dual Pixel CMOS AF in Live View. Draw
• The older Rebel T7i has an RGB+IR color light meter vs. the newer Rebel SL3's older technology dual layer meter. Advantage: Rebel T7i
• The older Rebel T7i has a slightly more powerful built-in flash that can act as a "Master" flash in the Canon optical wireless flash system. The newer Rebel SL3 has a slightly weaker flash that is not "Master" capable. Slight advantage: Rebel T7i
• The older Rebel T7i has an ISO compliant hot shoe that can manually fire any center-pin flash (that doesn't have a trigger voltage that exceeds what the T7i can handle) or flash trigger transmitter. The newer Rebel SL3 has a "borked" hot shoe with no ISO compliant center pin that makes using anything other than dedicated CAnon TTL flashes a pain, if possible at all. Advantage Rebel T7i

Personally, the borked hot shoe would be all it would take for me to forget considering the Rebel SL3 for anything. If you don't ever plan to use any external flash (in other words, if you don't plan on learning how to be a photographer instead of snapshooter) it won't make any difference.