Isn't CO leaving the Chase Sapphire family shortly? That would leave you without a way to consolidate your miles into UA/CO program. Your only airline transfer options at the moment are BA and CO with this card which makes it less palatable to me.

For the UA Explorer card make sure you will hit the spend thresholds to maximize the bonuses. IMHO their current bonus offer is poor. I would wait a while to see if they make it better. You have to spend $25k on the card in the first year plus add authorized users just to get 40k bonus miles.

I got in on the 2 AA cards with 75k bonuses a while back. Of course those miles are good for booking anyone in OW, not just AA. So I think it's worthwhile to collect miles in both of the big alliances at least.

You may want to consider a card like the SPG Amex that a lot of us like. Not a huge sign up bonus at the moment. But you can use SPG points on a lot of airlines. Unfortunately their UA transfer ratio is poor. Or you could go for an Amex rewards gold or plat card.

Of course it also depends on what you are trying to get out of this. Do you want as many miles as possible to fly international premium cabins? Or are looking to fly domestic Y?