I cannot speak for United/CO but I can speak for DGS and Mainline DL as I have worked for both - Mainline RR has better flight bennies then DGS...you would have a higher boarding priority as a mainline/RR with Delta plus you have your buddy passes which DGS frontline does not have...with RR you can only swap shifts with other RR employees so at smaller stations it can be more difficult to trade around - at my previous station we were 5days a week 5hrs a day and we would buddy up and work just 3 days a week (2 10hr days and 1 reg shift)...DGS employees have to apply for mainline jobs as an external candidate via delta.com and go thru the Peoplescout process where as RR you can bid on internal jobs via e-bid..