Since you have never played either GURPS or d20Modern, I'll offer this, having played both in WWII settings:
If you decide to try GURPS, consider using only the GURPS Lite Rules and adding on only what you absolutely need for your WWII setting. If you like it, consider using the full GURPS rules for your next campaign - and if rules-heavy isn't your style, you can stick with GURPS Lite.
D20Modern gives you a quick-start advantage because it's derived from the rules you already play. d20Modern is much more rules-light than full GURPS, and not nearly as light as GURPS Lite. You can use the GURPS source material with d20Modern with a little work.
I don't think you can go wrong with either system. Good luck with your game.