Is this for you 50g cube?
But, that really doesn't matter too much, IMO either bulb will work. BTAs ( any color ) don't require as much light as other hosting anemones, but do well under additional light as long as they are acclimated to higher levels of light.
I used to have a GBTA under a Pheonix 14K and it did great (( recently sold it ))
It is going to come down to which bulb your prefer, either one will work great as far as the RBTA is concerned.