These have been dealt with in other threads I suspect, in great detail.
But in summary, I use fondant because I can just stick it on from early September and not worry about them over-filling the brood nest with stores, as they don't take fondant down as fast as syrup (and they seem a little "calmer" about fondant than syrup). I also don't have to worry nearly as much about triggering robbing, as fondant has little "smell".
But syrup is fine and I have fed it lots in the past. The negatives mentioned above re syrup can of course be managed with correct application. I just like the simplicity of fondant feeding.
The main negative of fondant is probably that, as mentioned above, they take it down slower. So if you are looking to "feed to weight" in one go in (say) early October, it takes longer to do with fondant.