Window glass U-value 0.7 W/m² K vs. 0.75 W/m² K

Not worth it, I would say. Reason: You mainly have large glass surfaces facing south. Windows that insulate better also reduce solar heat gains. There is special glass for this (we have so-called "Solarwhite" glass) with a higher g-value that lets through more sunlight. I would also stick with the 0.7 glass and not take the 0.5 one. In the worst case, you have no energy savings but higher energy costs due to the "better" glass. The weak point is the frame anyway.