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 L3550. L3550 has ISO black print speed of 15 ipm, versus 11 ipm on L5390. L5390 supports an automatic document feeder (ADF), whereas L3550 does not. L5390 supports Ethernet, whereas L3550 does not. L3550 has maximum borderless size of A4, versus 10 x 15 cm on L5390.
The paired specifications favour L3550, 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.