Basic information:
Release date: MFC L 8690 CDW = 26.4.2017 - MFC L 8390 CDW = 27.11.2023
Operating system compatibility:
MFC L 8690 CDW = Linux, Mac OS X 10.10 (Yosemite), Mac OS X 10.11 (El Capitan), Mac OS X 10.12 (Sierra), Windows 10
MFC L 8390 CDW = ChromeOS, Linux, Mac OS, Windows 10, Windows 11 Home, Windows 11 Pro, Windows 8.1
Print resolution: MFC L 8690 CDW = 2400 x 600 dpi - MFC L 8390 CDW = 600 x 600 dpi
Time to first printout: MFC L 8690 CDW = 31 s - MFC L 8390 CDW = 12.50 s
Scanner features:
MFC L 8690 CDW: Optical resolution 1200 x 600 dpi - Scan format 216 x 356 mm
MFC L 8390 CDW: Optical resolution 1200 x 1200 dpi - Scan format 210 x 297 mm

According to the data sheet, the MFC L 8690 CDW should have a better print quality. It should be borne in mind that the release date of the MFC L 8690 CDW is 26/04/2017, i.e. technology that is almost 10 years old. You also have to consider the operating system compatibility.

They don't have the same colour cartridges - is that relevant? We use both, I don't notice any difference in everyday office life. Except that we have to buy two different toner cartridges.