Depending on the supplier, both the 60 and 70mm should come with 28mm units.
The slim outer frame enables you to improve the energy performance of the window.... Less frame/more glass = better efficiency
There are other ways to reach a,b or c rating ie frame thickness, glass and reinforcement type. Explaining energy ratings to customers isn't easy
http://www.astrasealdownloads.com/files/EnergyRatedBrochure.pdf
Performance wise they're near enough the same, looks wise I'd go for TRITEC, overall the 70mm wins, just feels stronger, especially the doors compared to the 60mm