In wtt the gold shine will be higher and more silvery colored. In black background the gold will be more intense and yellow, gold in color but a lot of the shine will be lost. The extent to which way the color goes depends on lighting. For xb, cool white shop lights will do. You don't want lights that are too intense if you are using wtt.
Mine are in black tank since I prefer a more yellow gold tone and don't care about how high the shine goes. My lighting is also more intense with 10000k lights coming on for 5 hrs / day. I am more interested in the overall look of the tank than 1 or 2 fish.
When I bought my first XB I was told if I wanted the shine to be higher that I should leave the XB in wtt until 14". I was also told that if you are willing to wait a few years the crossing will come naturally as long as the lighting isn't too intense. I hate the sterile look of WTT. I have seen fish that go from WTT to black tank that were gold head XB's, at over 14", that lost a lot of their gold shine and barely have any gold left in their head but the intensity of the gold color is amazing. It really is a matter of preference.