I would say that d20 Modern is sort of the "Microsoft Windows" of Modern gaming. It's not the perfect system for everybody, but the sheer weight of existing product, driven by market share, means that you can probably find or make rules to do what it is that you want it to do.
Spycraft2.0 is the "Apple" of Modern gaming. It's sleek, it's cool, it does the thing it does real well and it wants to do what Windows does on top of that. It doesn't have the market share of Windows, nor the depth of support or product base. It tries to make up for it with coolness and flexibility, doing alot of what you need add-ons to do with Modern, er, Windows with it's own bundled software. For a bundle, best-in-class, but you can find each individual piece of software in a more robust form on Windows.
Then GURPS is ... LINUX ... it's got the world at its fingertips, if anybody can figure out how the hell to use it.
So I'll put it this way ... I'd LIKE a Mac ... I HAVE a Windows box. I had a choice the last time I bought a computer, and I still bought a Windows box. When Mac is sleek and cool, Windows can sort of steal the thunder with a 3rd party product.
--fje