first of all don't listen to people telling you not to pick either because they are weak, basing your class choice on current meta is stupid. lost ark gets balance patches pretty frequently so bm and infighter being on the weak side will change.

that being said, the difference between them at their core is minimal. both have a good amount of cc, both require setup to get off their slow-casting main burst skill, both have a few mobility options, both are looking to isolate a 1v1 fight and pressure ranged characters. id say infighter has more survivability because they have more superarmor and immunity tripods, bm is faster with lower cds so a higher overall apm than scrapper who is a little slower. basically youre picking between being slower but tankier or faster but squishier.