Well, since your asking on the GURPS-forum, expect to get pretty biased opinions.
Anyway. The main differences are that D20 modern is a class-based system where once you take a class you get to onyl so what that class can do. This might be beneficial to new GM's and players. O nthe other hand it can feel pretty restrictive.
GURPS on the other hand is point-buy where you can construct any sort of person (or creature) you can imagine. For instance you can chose to only be good at fighting, or not be able to fight at all.
Another strong difference in character-making is that in D20 modern you start with an almost blank sheet and bare minimum character. The character are very much alike and only through earning enough XP for several levels do the character really start to differ and become able to "do something". GURPS makes you create a fully fleshed out character that are what you want them to be. Not "a potential to be what you want". So agian it depends what you prefer.
Then comes combat. D20 modern is highly abstract with little more than a curtsey nod to how the real world (or even Hollywood) works. Things behave by the rules, more than "how they ought to". Here GURPS is much better at representing a typical action-movie feel (unless it's really over the top action, but even then). It does come at the price of more
options though, which can be overwhelming and confusing even to experienced roleplayers.
I'm not going to advice which to pick, but I will say that you can get to play GURPS completely free by downloading GURPS Lite:
http://e23.sjgames.com/item.html?id=SJG31-0004