DEI has some new moduels now that are more capable of learning the OEM keys signal so you don't have to give up your key.

Some times with new cars though you just have to secure an OEM key in that box so it can transmit the correct code everytime the remote start is operated.

I don't work in auto alamrs anymore but I remember when doing installs of remote starts on luxury vehicles that was a huge issue.

If you have to give one up I'd give up the spare master and keep one master and valet. Then one day just order the new master from the dealer since you say they are both the same price.