Considering that d20 modern has almost zero support for sci-fi as written, I think I'd have to say Traveller.
That said, you will probably note several similarities. For example, both d20 modern and T20 lean heavily on feats, including specialized class feats in traveller which somewhat resemble d20 modern's "talents."
There may be a few things you might want to take from d20 modern, like wealth rolls. But overall, for Sci-Fi, d20 modern has nearly zero support beyond its fairly generic baseline.
I just got my T20 book the other day and it has some great rules. Despite it's hefty price, I'd suggest it to anyone who wants to start a 'realistic' sci fi game as opposed to a space opera game like Star Wars. Although if you have the extra cash, buy both Modern and T20 and patch both the games together. After all, that's what D20 is all about