I know this dog has been beat to death...
However my .02 - The synthetic line is safer because it doesn't hold kinetic energy like the metal line. It's also MUCH lighter if weight on the front end is an issue. Likewise, it's easier to move around, reel in, doesn't leave metal splinters in you if it rubs past your gloves up to your arms, etc.
Nothing is perfect. Synthetic line does need to be replaced periodically, especially if it's exposed to direct sunlight on the mount you're using. It's more prone to chafing and you need to make sure that the rollers are delrin and the snatchblock you're using hasn't been used with wire line in the past. If you're going to be winching over break-over rocks and doing a lot of rock work, metal line is the only way to go.
For most of the FJ type folks with the mountain driving we do (I'd say 99.8% of us), the synthetic line is better because it's safer and it's easier to use.
For the rock crawling crowd (not that many FJ's do SERIOUS rock crawling) metal line is better.
And while you're at it, get a safety thimbal on the end, don't use a hook. Shackles and the thimbal are much safer.