M209d and MFC-L5900DW share specifications such as Print Technology (Laser), Automatic Duplex Printing (Yes), USB (Yes), Maximum Paper Size (Legal).
The main differences: Printer Functions (M209d: Print, MFC-L5900DW: Print, Scan, Copy, Fax); Black Print Speed A4 (M209d: 29 ppm, MFC-L5900DW: 42 ppm); Print Resolution (M209d: 600 x 600 dpi, MFC-L5900DW: 1200 x 1200 dpi).
M209d may suit users who need its differing values, while MFC-L5900DW benefits those who need its paired strengths. Review the specs table above before deciding.