Well, I did write a review of both products.
But to briefly sum up:
OGL Cybernet is a complete game (rules wise, there's no setting), but is essentially d20 Modern, with some tweaks and additional rules for Cyberpunk, like rules for netrunning and cybernetics. IMHO, the class system used in this is a bit broken. Basically, it comes up with 6 new classes to replace the 6 base classes from d20 Modern, in some cases just tweaked versions of the classes. Which would be fine, except I don't think it covers all the notable archetypes of cyberpunk. There's no class for ganger or street samurai.
d20 Future is basically just an expansion for d20 Modern, essentially a great big toolkit.
Honestly, neither is great.
I like the cybernetics and netrunning from OGL Cybernet. But the rest is kinda 'eh' or just plain old d20 Modern.
d20 Future does an okay job on most things (except starships), but just glosses over most things. Broad but shallow coverage. (and the netrunning rules aren't in the book, they are in a web enhancement)
So, I would say, buy both used. I know I got OGL Cybernet for around $20, and I would have to think you could get d20 Future for the same.
THough if you don't have D20 Modern, then you should probably just get OGL Cybernet, since in order to use d20 Future, you need it and d20 Modern.