The first thing I'd consider, is Raelin's popularity vs Kelda's. As Raelin has become a much more popular tournament unit than Kelda, she is the logical choice. The fewer order markers needed to use Braxas is a major advantage, as she can then use those turns to acid spray.
However, with the right luck, Kelda can obviously extend Braxas's life indefinitely. With 8 life, she should have enough time to do damage and get back to where Kelda is to heal.
Then again, if you are matching Braxas up with the Greenscale Warriors, Raelin would most certainly be the better pick, as you could then potentially have several Greenscales with 6 defense, and Braxas with 5 defense. Thus in conclusion, I must say that Raelin is the better choice (even if it is the less original or exciting choice).