I’ve worked for both. I quit Atlas 7 years ago so my experience probably won’t totally match the current conditions there. Both companies have very similar operations. Gateway/Home basing. Constant schedule changes. Occasional long layovers in cool places all over the world. Augmented crews just about every leg. And a mix of odd/fun crew members.

Now the differences.. Hourly is currently better at K4 and the schedule is better. 14-16 day lines where Atlas as far as I know is still 17 days across the board. Also Atlas can extend you where K4 cannot. This is huge IMO. I haven’t seen the current contract at Atlas but from I hear they have slightly better work rules at the moment. The management is better at Atlas. Travel and scheduling felt more efficient where K4 is often dysfunctional. I get the feeling that K4 still operates like this small mom and pop airline. More variety of airframe at Atlas which gives you more options in the kind of flying you want. (Domestic and sleeping regularly vs international zombie)

I value time at home above everything though so if I get 3 more days with my family I’ll deal with the dysfunction. But if I lived in a Atlas base that would also weigh in heavily.