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
Each system is different My Audio Logic DAC sounds sweeter with RCA digital cables then the XLR ones but there is s slight decrease in detail So try them for yourself

Happy Listeing
Are you saying the Wadia will lose clocklink with XLR? I was under the impression that clocklink will work with other connections. Am I wrong here? Of course I will leave the AT&T between the units for the clock signal. Maybe I need to go back and re-read the manuals.
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.