It would be helpful to have the context, because in some contexts there is very little difference. For example if you were asked the question 'What do you do in your free time?' it means exactly the same as 'What do you do in your spare time?' - want the person wants you to communicate is what you usually do when you are not working. However, in some contexts there is a difference. For example on a guided tour (holiday/vacation) you might see the phrase 'Monday afternoon - free time' used to indicate that there is no planned activity on Monday afternoon. If you used 'spare time' to indicate this then people would probably understand but it just sounds wrong.

None, they are basically interchangeable.

I agree that both 'free time' and 'spare time' can be used interchangeably.

Both can mean the same thing. The only difference would be that spare time implies you are always busy, and you only have a little bit of time to spare to do something. Free times implies you have a bit more time on your hands, maybe larger chunks of time that you can do more things, or that you can enjoy doing something more.