So here am I, who have never flown either UA or CO, being asked for advice from a friend in the same boat! She has always flown AA from STL, but is about to fly CO for the first time. Before the merger was announced, she thought she might fly UA sometime, so signed up with them. NOW what should I tell her? Get a CO account instead?
Nah, just have her credit her CO flight to her UA account. Better to have what meager miles she has consolidated in one place. Easier to keep track of and reduces the risk of losing miles during program consolidation (not that I expect that to happen, personally - the losing of miles, that is).