Chase has been offering 10K bonus miles retention for UA Explorer card. However, they have not been offering any decent retention offers for CSP. Check the Chase retention thread for details. Also, if you spend 25K in a year, there is a 10K bonus for UA card. My UA card came up for renewal a few months back and got the 10K retention and will soon get the 10K spend bonus as well.
If your primary usage of UR points is for united, it might make more sense to keep the UA card.