The two don't have to be mutually exclusive. We can all each have a set fate, yet make descisions that can change how we get there.
The easiest example I can give actually has to do with the editing equipment I'm sitting in front of as I type this (well, OK, technically it's off to the side of the computer I'm sitting in front of...).
The fate of the video I cut is to have a certain set of shots of specific length and conforming to proper video and audio standards. It has to end up that way to meet the requirements of the show I'm working on. It would seem there is only one way to get to that outcome.
The opposite is true, though. The manufacturers of said equipment have seen to given me the God-like ability to pick any one of 10 ways to cut the shots together, color correct them and adjust the audio. It will all end up looking the way it's supposed to, but I can pick any route to get there.
That's how I see the mythology of "Lost" being. You have a set eventual outcome. Charlie was destined to die, for example. However, there appeared to be several alternatives presented by his and Desmond's actions to lead to that result.
So, the characters' fates are all sealed. There is an eventuality and purpose to each person. However, they can alter the path they take to get there or outside forces can do it, much like water flowing down to a stream. A small rock or mound of dirt can alter the flow of the water, but it eventually gets to the stream.