Thinking about an upgrade

I recommend avoiding the 18 mp sensors. Not because they are bad but because Canon has made some major improvements to their current 24 mp sensors. Since you appear to keep a camera for a long time it seems you are justified to look at the T7i or the SL2 to provide the best longevity with the best technical capability over the time you will use it. The main difference between these two cameras is the auto focus system used through the optical viewfinder. The T7i is more sophisticated and is geared toward shooting fast moving subjects. The SL2 has an older system but its viewfinder is larger and brighter as a trade off but it is a very capable system also. There are smaller differences but more minor. The SL2 is smaller and lighter weight than the T7i. Overall though, they both have the same internals with a few more external controls being added to the T7i. I also recommend that whichever one you choose that you get the kit lens too. Canon has made many improvements to the 18-55mm kit lens over the years and it is a great addition for the small additional cost increase over buying the body only. Especially if you are still using the 18-55mm that came with your XSi.

Both of these cameras are available on Canon's refurbished website (click for link) and if you are patient you can get one for a great price. I have bought refurbished gear from Canon before and have never been dissatisfied with anything I purchased. The best time to buy is around holidays. Canon recently had a fantastic deal on the SL2 where they offered an additional 15% discount for Saint Patrick's holiday. They run the 15% discount fairly often so being patient and checking daily will payoff at some point.