Open Source: You can analyze the source code and build it yourself - which is great if you don't trust anyone to give you binaries from what you analyzed. Usually at least free as in "free beer".
Free Software: Open source software which gives you lots of permissions via its license, while making sure you get to keep these permissions. Usually free as in "free speech" (in addition to "free beer").
TrueCrypt not being free as in speech is a bummer, but being able to inspect the code and build it yourself is a critical advantage, especially when it comes to cryptography. I have a hard time imagining BitLocker not having any backdoors built in. At the very least it'll have some kind of weak random number generator or whatever, making sure that with the right algorithm you get to crack it within a few minutes or so.
