Glass Fiber AT&T ST or AES/EBU XLR ?

What and how large are the improvements (assuming there are some), going from AES/EBU (XLR) to Glass Fiber (AT&T ST) digital cable, between Wadia 270SE & Theta Gen' VIII?
Is the AT&T ST worth the $300 upgrade on my Theta Gen' VIII?

Thanks, Tomer
RJA yes clocklink only works by using both AT&T cables. 1 cable has the clock data the other has the CD word data only. Believe the balanced cable has both the clock and CD information combined into it.
Just received an e-mail from Keith at Wadia. The clocklink works with any cable. Of course the clocklink must be connected by the AT&T glass cable and transport and DAC have to be configured for clocklink. As long as this is connected and active all other cables no matter which termination will carry only the word data. This is as I suspected but I was not positive. Thus the confirmation from Wadia.