...but they are a bit different in a number of ways. I've never had the 80D, but in general, the EOS M line of cameras will be somewhat slower. On the other hand however, the size/weight factor more than makes up for the weight lost from shifting from a DSLR. IQ-wise, they are very, very similar.
The EOS M100 is purely compact (begs the question as to how in the hell Canon managed to fit an APS-C sized sensor in there). With the standard kit 15-45 can fit in some pockets. It also had DPAF capabilities which give it much faster and accurate AF in many scenarios as compared to the earlier EOS M3 will. Additionally, the color tonality of the M100 is more on the warmer (more traditional) side of the house - more like what we were accustomed to with past Canon cameras. The M100 does not have an external hot shoe, so you are left to shooting with the internal built-in one only.
The EOS M3 is a tad larger and thicker than the EOS M100. It is capable of superb IQ - plus the ability to attach an optional EVF on the top (of which Canon currently has two available) gives you the option of shooting/composing with an EVF. Take off the EVF and you have the ability to attach an external flash. Color tonality-wise, the colors coming from the M3 are somewhat different (not bad, mind you - just different), with some folks characterizing the colors shifting to more greenish tones in certain scenarios (where yellows should be).
I still use both. On the M3 I've attached the EF-S 55-260 via an adapter and the combo is superb. On the M100, I keep a very good copy of the 15-45 and use it where having a very compact (low profile) camera is a necessity.
I think you would be happy with either one. After all, they both feature 24 MP sensors, with the one in the M100 being a newer version. In the end, price often rules, so if you can get a new (or superbly used) M3 w/kit lens at a great price (kit lens for the M3 usually means the older yet very competent 1EF-M 18-55), then that may be the supreme motivator. Likewise, I'm now seeing the M100 with kit lens going for less than $400 USD.
Here are some quick comparisons:
EOS M3 sample shots: Please note that I use the excellent image conversion program called "Irfanview." However, when I create JPG's from my TIF masters, the exif data is striped with this program

ISO 100

ISO 100

ISO 5000

ISO 100

ISO 1600
EOS M100 Sample Images:

ISO 6400

ISO 4000

ISO 400

ISO 100

ISO 100

ISO 100
Good luck...
--
Life can be good - if you allow it!
Bernd ("Ben") Herrmann
Fuquay Varina, North Carolina USA