Both printers include colour printing, Wi-Fi connectivity, and USB connectivity. Their most relevant distinctions are fax and ISO black print speed.
L5390 offers print, scan, copy, and fax, versus print, scan, and copy on ET-2840. ET-2840 has ISO black print speed of 15 ipm, versus 11 ipm on L5390. L5390 supports an automatic document feeder (ADF), whereas ET-2840 does not. L5390 supports Ethernet, whereas ET-2840 does not. L5390 has maximum print resolution of 5760 x 1440 dpi, versus 4800 x 1200 dpi on ET-2840.
The paired specifications favour ET-2840, particularly on ISO black print speed. The other model may fit better if its own strengths matter more to you. Consider the first model if fax matters most; consider the second if ISO black print speed is the priority.