What @almarg said -- the deciding factor is what the impedance is on the device you are connecting the clock too -- I've never seen a device with clock inputs that has both 75 and 50 -- it's either one or the other and that's what you need to drive.
By the way a poorly performing clock interface can be very odd to track down -- sometimes it just fails to work which is clear but other times problems can manifest as distortion on transients and you can spend hours seeking other causes of the problem in your system before realizing it's just that the clock has gotten mis-synched. That's all to say don't mix 75 and 50 or you can cause yourself a world of pain!
By the way a poorly performing clock interface can be very odd to track down -- sometimes it just fails to work which is clear but other times problems can manifest as distortion on transients and you can spend hours seeking other causes of the problem in your system before realizing it's just that the clock has gotten mis-synched. That's all to say don't mix 75 and 50 or you can cause yourself a world of pain!