As a UA FF out of a non-UA hub (PHL), I'll give some benefits I have. (I'm not sure if it is the same at ATL).
1. UA flies out of one terminal at my airport, if I fly the hub airline, I could be anywhere in the airport complex and returned to anywhere
2. I ASSUME I get more upgrades clear (CPU, RPU) because there is less competition in theory
3. It is easier to earn more PQM because you will connect more and can just connect further or take advantage of the 500 PQM minimum, maybe you can get to 1K
4. I think UA offers the most E+.
5. I think UA miles are more valuable, I've heard DL miles referred to as sky pesos
6. I believe UA has a better intl network
7. UA's polaris products (seat, lounge, etc) is probably better than DL currently
Cons:
1. Although you can earn more PQM, You have to connect more
2. I am near EWR, a UA hub, if I want to go direct somewhere far, for you, you don't have a UA hub near you so you dont have that option
3. If I recall, the UA checkin counter is quite far away
4. UA miles can expire, DL miles do not
5. Does DL have a million miler type of program? You'd be starting over on UA
6. Connections means your upgrades will be on shorter flights with worse service
7. You may end up on smaller planes with shorter routes
8. UA does not count bulk tickets towards PQD, I thought delta had a more generous policy
9. DL allows you to pre-order meals (if I recall), UA does not
Depends what is important to you