FreeBSD is the most common choice because it is good with security, and yet still flexible (and very stable). OpenBSD considers itself the most 'secure' of the BSD's (which means it can be a pain to do some things with), and NetBSD can be run on the most kinds of hardware (and the base install is *very* small, which can have it's ups and downs).
But in general, people go with FreeBSD (which is what I do).
As for installing it on a machine with windows and redhat already, I couldn't tell you about that. I gave up dual booting (triple booting in this case) long ago, I just give it the whole system when I'm putting on linux or *bsd.
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