Free functions, whenever possible (and practical), in my opinion. At least with that option, you can group all the functions into one header (if the left-hand and right-hand arguments are of different types); otherwise, you'll probably get one operator defined in one header and the other in another header. I prefer to keep my operators logically grouped together, which cannot always be done with member-function operators.
To expand on what ParadigmShifter mentioned, if you have operator* that multiplies a scalar (i.e. float, double, etc.) and a (math) vector, you'll probably have two operator* functions (so you can do (#1:) 3 * vector and (#2:) vector * 3). #1 cannot be a member function, but #2 can be. I like to group these two operators together, because the code for each of them is pretty much exactly the same (and they both do the exact same thing). The only way to group them together is to make both of them non-member functions. And for consistency's sake, that means I try to make all my operators non-member functions (or at least all that are practical to make non-member functions).
Some things, like operator=, cannot be a non-member function. Functions like these are, of course, implemented as member functions, because they legitimately need to be.
Edited by Cornstalks, 22 June 2013 - 05:36 PM.