imo fx makes more sense. You are already changing to spring sponge and it's going to feel very different from other rubbers you played. If you also change the hardness by a lot it's going to be tough to evaluate the tenergy. Compare the softer 05fx to the 38 - 43 degree rubbers you have been using up til now.
A month ago, before I switched to T05 for my BH, I, too asked the same question, that is to say, T05fx, Rozena or T05?
My coach answered, " Don't be a pussy, use the real stuff, T05. The only rubber to spinz them all "
Caveat: Your milage may differ and take my post with a huge pinch of grin. Happy New Year. 😁
p/s My coach represented my country during the 90's and he played with Sriver ( during the speed glue era ) and then switched to T05 throughout his pro career. It is only recently he switched to Dignics05 ( he gets is free 🤬 ) . Me thinks he is bias... 😁