If you have any kind of status the Explorer is rather worthless IMHO. I would never choose to give up my Select, but there is also nothing to prevent you from getting both. I have both the Select and the Club Card.

The Select simply earns far more miles at a much faster rate. 3-1 miles on UA purchases, 2-1 on *A, gas, dining, groceries, and home improvement, and 1-1 on everything else. In this 1-1 situation I switch to the Club which get's me 1.5-1. Plus the Select get's you a 5k annual RDM bonus and up to 5k in EQM on UA purchases.

Since you can't get a Select anymore I would recommend simply adding the Explorer first to see if the CPUs on award tickets are really worthwhile rather than giving up the Select all together. Even as a lowly silver I'm also not really a fan of what UA has been doing with the Explorer as a way to buy status, which the card has devalued even more now for us BIS miles folks with the new boarding groups. As a lowly silver, I can take this to a degree, but I feel especially sorry for the grief the card is now bringing to golds and above.