The spacers are included in the WERS Uw rating, so I would only look at the Uw rating.
It is true that a "warm edge spacer" is more efficient that the standard aluminium spacer, but taking into account the impact of the spacer in the overall Uw, the window design is far more important.
The recorded difference between these would be U 0.1 Watts/m2/oK. But as I say it's built into the Uw that is published in WERS.
Our standrad double glazed window with clear glass, beats many thermally broken windows.