Well, IAD is a UA hub. If you like your flights direct, that's a good thing for you.
If you only have 2 Chase cards, I'm sure that it will not be hard to get a MileagePlus Explorer. The bonus is very high right now. 50k first spend, 5k authorized user, 10k if you happen want to spend $25k in a year on it, and $50 statement credit.
UA miles are so much more easily redeemed at low tiers as well when compared with DL miles. Good luck finding domestic flights for 25k round on DL

32.5k to 40k typically if you are not very flexible with your date/times.