Pronunciation of Gayatri Mantra (सुवः vs स्वः)

[Vivek Rallabandi]

Pranams to the esteemed scholars present here.

I have come across two versions of the Gayatri Mantra wherein the following difference exists: (1) भूर्भुवः सुवः or (2) भूर्भुवः स्वः 

What is the reason for this difference? Does the variation depend on one's veda-shakha?

I belong to Krishna-Yajurveda, Taittiriya Shakha — based on that, which one, in particular, would be applicable? 

With gratitude,

रा. विवेककृष्णः