Dial Global was always very good to me when I was OM at a station that affiliated with their then Bright AC format (Bright AC and Jones Hot AC merged in 2008). The PDs were always willing to listen and help me out as much as possible...even giving me tools and resources to use when I had no promotional budget. This was all on barter.
DG is pretty flexible in their affiliates needs in general, from what I've experienced. If you want to be an affiliate of their Hot AC 24/7 format, but also want produce a music log for "live" or "voicetracked" hours or if you want to move away from the 24/7 format for a hotter or liter music direction during different dayparts, they have a "Custom" service where they can supply you with custom tailored logs complete with your own elements set to your hourly clocks at no charge. They also have the "Local" service, but that can turn into a barter/cash situation with the more local elements you want them to do for you (like weather).
The only experience I have with Citadel as an affiliate was with their Oldies Radio format when they were owned by ABC. Where the product was fine, they were never flexible to the affiliates needs. Jingles, websites, and promotional tools all came at a fee, the affiliation was cash/barter, and liners were free as long as you didn't go over your quarterly quota. Again, this all may have changed over time.
Musically, both services are roughly the same, but DG seems to bring out more gold "fun pop" from the 80s and 90s than Citadel, especially during their retro shows on Saturday nights.
In terms of elements, DG is very flexible. You can either use their own liners, jingles, and promos or use your own 90% of the time as they darken the satellite feed for all local elements. The only real exception is when they're contesting and when they use the tag line "The Best Mix of the 80s, 90s, and Today" (Although, that may have changed since the merger). Citadel usually has a lot of their own liners, jingles, and promos on their formats, which may give your station a dual identity unless you conform to how they promote themselves.
There are plenty of options, and its all a matter of what you want to do and what your station & market needs happen to be. If you want a fairly hands-free service, Citadel is probably the best choice, but if you want more control over your station, then DG is probably the best route to go.