Upgrade from 100D to 800D....or new lenses....

That's hard to say, not knowing exactly what type of pictures you take and what you expect to get from them.  But I can say for me, that there was a big jump in improvement between my Canon T2i/T3i (550D/600D) cameras and the Canon T7i (800D) camera.  The Canon SL1 (100D) camera was similar to the Canon T3i (600D) camera, both using a 18mp sensor etc.  Eventually, I'll probably get another Canon T7i (800D) camera to replace my Canon T2i (550D) camera.

It was a bit different but last fall I upgraded from a t3i to a 77d which would be somewhat similar and I have no regrets.

I was able to sell my t3i for decent price which made upgrading more affordable.

You can read the specs yourself but here the things that make the most real difference to me.

1) The noise is a LOT better on the 77d.  It may just be me but even at ISO 800 I can barely see any noise so I am routinely shooting at higher ISOs now,  You should be able to find images on the web to see how this looks to you.

2) The burst rate is a lot better even in RAW and I use that a lot more to catch action shots. On the t3i I could only take a burst of about 3 RAW photos before it paused which was frustrating.  I don't think that I have hit the burst limit on the 77D where it paused.  The speed of your SD card does matter on this so use a good card.

3) I did not expect it but I love the touch screen and the feature where where you touch the screen to show it where to focus and optionally automatically take a picture too.   I use that a lot.

4) When I am not using liveview I find that I usually use a single focusing spot with back button focus so I don't use all the focus points that it has very much.  The liveview focusing system seems to be completely separate from the normal focusing system. That is mostly just me and the way I shoot.

These may be specific to the 77D but you might want to check these out too;

1) There were a lot of mixed reviews on inexpensive third party batteries so I ended up biting the bullet and buying the more expensive Canon brand battery for a spare.

2) I shot raw and with the t3i that worked fine with an older 2014 version of Photoshop elements.  The 77d uses a different version of the CR2 RAW file which it could not use so I ended up upgrading to the 2019 version of Photoshop Elements.  Canon provides free DPP software which is actually pretty good for working with RAW but that would require a new learning curve.

That 100d must be getting pretty long-in-the-tooth by now. How old is it?

Probably time for a newer body.

Like Watty above, I like the 77d. It packs most of the power of an 80d into a Rebel-sized body.

Watty dp wrote:

It was a bit different but last fall I upgraded from a t3i to a 77d which would be somewhat similar and I have no regrets.

I was able to sell my t3i for decent price which made upgrading more affordable.

You can read the specs yourself but here the things that make the most real difference to me.

1) The noise is a LOT better on the 77d.

That’s more thatbthe T3i is old

It may just be me but even at ISO 800 I can barely see any noise so I am routinely shooting at higher ISOs now, You should be able to find images on the web to see how this looks to you.

2) The burst rate is a lot better even in RAW and I use that a lot more to catch action shots. On the t3i I could only take a burst of about 3 RAW photos before it paused which was frustrating. I don't think that I have hit the burst limit on the 77D where it paused. The speed of your SD card does matter on this so use a good card.

3) I did not expect it but I love the touch screen and the feature where where you touch the screen to show it where to focus and optionally automatically take a picture too. I use that a lot.

4) When I am not using liveview I find that I usually use a single focusing spot with back button focus so I don't use all the focus points that it has very much. The liveview focusing system seems to be completely separate from the normal focusing system. That is mostly just me and the way I shoot.

These may be specific to the 77D but you might want to check these out too;

1) There were a lot of mixed reviews on inexpensive third party batteries so I ended up biting the bullet and buying the more expensive Canon brand battery for a spare.

2) I shot raw and with the t3i that worked fine with an older 2014 version of Photoshop elements. The 77d uses a different version of the CR2 RAW file which it could not use so I ended up upgrading to the 2019 version of Photoshop Elements. Canon provides free DPP software which is actually pretty good for working with RAW but that would require a new learning curve.

Ah, that Canon 90D looks sweet.   I wonder if Canon will make a 32.5 mp Rebel?  I prefer the lighter cameras, even though I use a relatively heavy telephoto lens.