My son flies EWR-ATL a couple of times a month, so here's my $.02:

IMO, CO seems to have a better on-time record for evening flights to ATL. Not sure if this is statistically true, but everytime he flies DL, things are delayed, and often flights are cancelled. The CO Friday night flights around 6 pm are often overbooked, VDB is usually about $200. Not sure about the ATL end, since he is flying as a UM, and they don't bump them.

CO flights to ATL depart out of Terminal A; DL is in Terminal B, which seems to have longer security lines, which could be a problem if you are not elite. The PC in Terminal B is also convenient to the gates, if this is an option for you.

EWR is CO's hub, which in my opinion means better staffed. Delta check-in also seems more laborious to me.