Not chastising you, however, it is U-Factor. Just in an effort to maintain informational accuracy.
As far as 0.30 vs. 0.26 goes, one is 15% more efficient than the other. Will you feel that, maybe not. Will you energy bills see the difference, as long as all other factors are comparable, yes.
What I mean is that if the other performance measurements are close, the window that is 15% better at blocking energy movement will save you money. If the window that is 0.26 U-Factor does so while lowering your SHGC to 0.20 vs. 0.30 in the other window, it might be a wash because you will get more passive solar from the sun in the winter months from the other window.
Does one leak more air than the other? Does one sacrifice Visible light? All these play a factor.
If you can get all the performance data together, that will help.