Do both. They'll be in different alliances.
Use fare compare (depending on where she is located) to find great fares, and do the MRs.

1. CO Gold is worth 100% bonus, and upgrades are complimentary based on availability 3 days out. Double mile promo is worth it. You also get fed. CO Gold will be Star Alliance ELITE PLUS, and that is also worth it. Don't bet on fellow *A airlines doing upgrades, however - UA wants upgrade certificates and US, while giving their own elites upgrades, doesn't appear to do so for other alliance elites.

2. DL Silver is worth it for Skyteam, but will be Skyteam Elite (not Elite Plus). 25% bonus for Silver, but DL is generous on promos and MQM offers, though no double MQM promo right now (except 2-night Hilton stays thru 10/31 - use it). Advantage is that any miles over the Silver Elite threshhold will roll over to next year qualification. (none below if she doesn't make it). Complimentary upgrades based on availability 1 day out. GoGo internet codes abound.

If she really must choose, I guess I'd lean to CO, based on the extras, but it is ultimately up to her, and if she plans any international travel.