The M5 has a more direct access to various controls... Look at photos of the top and back of both cameras and you'll see (the respective reviews of each model on dpreview.com have some good illustrations). With the M5 some key functions can be done "by feel" while keeping your eye to the viewfinder (such as Exposure Compensation or changing aperture/shutter speed, depending upon mode). With the M50 you'll need to take the camera away from your eye to use the LCD screen and go into the menu for more adjustments.
The M5 uses a slightly larger battery and gets a few more shots per charge.
BOTH cameras use similar Dual Pixel autofocus. However the newer M50's has been expanded with additional AF points that cover more of the image area (up to 121 points & 88% coverage versus 49 points/80% in the M5). The newer M50's AF is also rated to be able to focus in slightly lower light conditions (-2EV... versus -1EV in M5).
Both cameras use a Canon 24MP APS-C CMOS sensor.... image quality should be pretty similar. However since the autofocus pixel pairs are embedded in the image sensor (and the M50 has 2.5X more of those), their sensors must be different in some respects.
The M50 has 4K video and up to 12800 ISO in video mode (expandable to 25600). The M5 is HD video capable, with up to 6400 ISO.
The M50 uses a new CR3 RAW file type (be sure any post-processing software you might want to use can handle it). The M5 uses the original CR2 type RAW file.
The M50 is one of the first models to use the new Digic 8 processor. The M5 uses the Digic 7. The newer sensor is probably the reason the M50 has slightly faster continuous shooting rate (10 fps versus 9 fps.... or with continuous focus 7.4 fps versus 7 fps). It also likely contributes to the higher ISO in video. The newer processor in the M50 appears to be complemented with a slightly larger buffer, too... rated for 33 JPEGs versus about 26 JPEG in M5.
The M5 has a slightly larger rear LCD, but it can only be tilted upward or flipped downward. The M50's screen can also flip off to the side ("selfie" style).
The M50 was intro'd in Feb. this year. The M5 is now getting close to two years old and there are some rumors that a "Mark II" will soon be intro'd. That's just a rumor though.
Yes, the Canon EF/EF-S to EF-M adapter is rather pricey at $200. The third party lens adapters are a good deal less... more than suggested above. Vello's sells for $60 and Fotodiox' for $40 (so actually $140 and $160 less, respectively).
https://www.bhphotovideo.com/c/search?sts=ma&ci=3420&fct=fct_camera-fitting-male_1595%7ccanon-eos-m%2bfct_lens-fitting-female_1596%7ccanon-eos&cp=15293%2b8470%2b3420&Ntt=canon%20EF-M%20adapter%20EF%20ef-s&Both cameras are currently being offered with $150 instant rebate, body only. Interestingly, the M50 with EF-M 15-45mm lens is being offered at $200 discount, but the M5 is not. It's being discounted the same $150 whether with lens or not. So with M50 you can effectively get that lens for $70... while with M5 it costs $120 more than body alone. Go figure! A more expensive M5 kit bundles it with the EF-M 18-150mm lens. While there's a two lens kit alternative offering M50 with the 15-45mm and EF-M 55-200mm instead.
Overall... IMO, video capabilities aside, M50 looks like a good value with incremental improvements in a few respects, but somewhat less immediate control access. If video is an important consideration, the M50 has a more obvious edge.