Re: GURPS or Modern D20?
I really really do not like d20 modern . . . If you want to play a d20 modern guns based game dredge up a old copy of Star Wars d20 and file the serial numbers off. Its not that I dislike class level systems to much (huge Earthdawn fan and I like forms of DnD) but the classes in d20 modern are boring and counterintuitive. d20 Star Wars handles it far better.
I've never run or played in a strictly modern GURPS game just modern mish mash so I have no specific insights on it. It works well for modern mish mash though. One peculiarity of GURPS is body armor is very important and rifles hit a lot harder than pistols
You might also consider giving D6 Adventure a whirl. Eric Gibson of WEG released it as OGL so you can get it free
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