Just a thought that came to me last night at about 4 am, while up for different reasons.
People who can y/b/m, both co and ua, are being encouraged to book co right now, as they can (often) instantly upgrade. OTOH, people on non-y/b/m fares are being steered to ua, both for a higher chance of ELR(especially with the new sale policy), and the (seeming) better chance of an actual EUA. Not sure how this changes for people doing international Y, but I'd lean towards UA metal due to the e+, especially if row 32 is booked. I wouldn't be surprised if CO/UA is really doing some deep analysis on people's patterns with that, to get metrics and generate policies going forward. I know my leisure travel would be on UA metal, vs business travel on CO. I tend to be a last minute B+ fare purchase, often TPAC, so I have to be a profitable customer.
Thoughts?

The vast majority of customers book by schedule and price and everything else comes WAY farther down on the priority list.

While I agree with SBM that for the vast majority of people its fare/schedule, I think this type of cross carrier behaviour is being deeply analyzed as we speak, since this profits an opportunity for incremental profit. Maybe not this exact question, but I'd wager a bottle of wine that they are looking at the travel patterns (routes, fares paid, days in advance booked etc.) pre and post merger. In this day and age, this data is there, and it just takes a clever mind, some computing power, and a manager asking questions like you raised to mine that data effectively.

I guess my point was that for domestic travel I'd currently be booking on UA metal, over CO metal, in any case I was not able to book a b/m/y up, schedule and price being similar and/or workable. With the same day buy up program, as a plat I have incentive to book UA metal on discount fares, even being CLE based. Even for International travel, I would seriously consider UA over CO, as the IFE isn't that big a deal for me, and the E+ is really worth while. Especially if I can not get row 32, why should I fly CO metal?