Looking at the Sun and Marvin websites, the 'whole-unit' NFRC values both companies provide for their typical casement windows is 0.38 U for Sun and 0.33 U for Marvin, so Marvin is a more energy efficient window (0.50 U is the government minimum -which is way to high, IMO). On a cold winter day, I can stand beside a U 0.50 window and a U 0.33 window and feel the difference.

Both companies probably have the same low-e, argon fill glass technology (they typically buy their glass from someone else), so the difference in U values is in the design and materials used in the window frame construction. IMO, there is a difference between companies that know how to do things right and companies that want to make a copy and sell it cheaper. Since windows are the single biggest energy gain or loss in a house, I wouldn't sacrifice here; I'd go with the Marvin windows.