Hi,
In terms of potential energy - NADH is a higher energy species than FADH2 as it causes more protons to be pumped into intermembrane space.
I think it would be reasonable to say that FADH2 has greater electron affinity in this way as FADH2 causes less protons to be pumped into intermembrane space than NADH, leading us to conclude that there is less energy release involved with FADH2 losing electrons, and therefore it's affinity for electrons would be higher.
Hope this helps!
Katt