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-3700. ET-3700 has ISO black print speed of 15 ipm, versus 11 ipm on L5390. L5390 supports an automatic document feeder (ADF), whereas ET-3700 does not. ET-3700 supports automatic duplex printing, whereas L5390 does not. ET-3700 has replacement black ink yield of 7500 pages, versus 4700 pages on L5390.
The paired specifications favour ET-3700, 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.