Don't really think either is better.......
That said, if I wanted safety and a secure hold of the knife then kydex (if that's what you mean by synthetic) is awesome for that, and I personally love kydex sheaths and use them on most of my synthetic handled knives, I do this also because it looks less out of place with these.
But, to be honest I like the aesthetics of a nice leather sheath and most of my favoured using knives and all of my wooden/natural handled knives are sat in leather.
(Can't think of owt worse than a nice puukko or similar choked up in digi-camo kydex pants!)
In the unlikely event of being made to chose then I would go for leather, 'cos I'm a tart like that!