Hello Krigel
My view:
DCP-L8410CDW: higher, footprint but the same / higher capacity toner in from the start / re-purchased toner with large capacity (6'500) and not only (3'000) / larger design = better cooling + less "stress" for paper (ex. corrugation) / optional additional tray possible / slightly faster / drum life almost double / no chip on the toner / ...
DCP-L3550CDW: More suitable for "low volume printers", e.g. 500-1'000p./year / nice and quiet in "silent mode" / compact / ...
In the end, it all depends on the requirements/use.
Greetings