50 ohm is for your function generator and test and measurement circuits.
75 ohm are for video feeds.
I don't know your application, or your scope.
Most scopes have 50 ohm input termination inside the scope. If you get screwy
readings going direct from the function generator to the scope, you may need a
50 Ohm in-line termination. I frequently have to use one going from my function generator to my DVM. My scope has a 50 Ohm input already.
Just one more thing to be aware of when you're taking readings.
You may or may not need it, I don't know your application.
Good luck.