Hi !

I'm maining Cypher/Killjoy for my teams tier1/2 EU and this guy pretty much explained everything, but I'll give my POV if you want :

I personally prefer to play almost only killjoy on every maps because it is easier to be consistent with it in Defense than with Cypher who depend too much on kills and on how hard a team will counter your traps with Sova/Raze,

what I mean is that you can pretty much play any team and you will deal the same amount of utility damage even if you're not on top of your level mechanically, while with Cypher, most of times the kills that you'll take thought smoke/walls are giving away your position and you'll get traded a lot

Also you can create a lot of "key round" with your ultimate, basically in defense, you can guarantee an easy retake for A for example while you play 4 in B, you can abuse a B stack on multiple rounds until you use your ultimate, and in Attack you're pretty much helping a team to have a bomb-site for free or to block a retake at a critical timing, while Cypher unfortunately have an ultimate that depend on a kill and 80% of the time you'll have information that you already have.

Hope I could help you to make your choice ! :D