According to Papazian (I think it was him... I read it maybe a month ago), fruit without sweetness is rarely a desirable flavor because it tastes out of place or weird to humans who are unaccustomed to unsweetned fruit. So if fruit is added to the primary, it's usually done with a very malty beer where the residual malt sugars make up for what is lacking in the fruit after fermentation. I am guessing the flavoring also has sugar in it? If so, then I would assume the same principle would apply and the result will be that any sugars in the flavoring are going to be consumed by the yeast and the beer won't be as sweet.
Putting it in the secondary is a good idea on one hand because it will give the flavoring more time to settle and blend with the beer. I assume it's a natural flavoring, not some artificial junk. I assume this means that there are components in the flavoring that could fall out during your secondary and lead to a more authentic flavoring. However, as someone above said, it limits your ability to "flavor to taste".
Then, there's putting it in the keg or priming bucket. This will obviously give you the most intense cherry flavor and will allow you to add
just enough to suit your tastes. My concerns would be incomplete mixing and the possibility that the cherry flavor will seem "added in" to a more experienced pallate, like an afterthought, rather than an integral part of the brew.
So my vote is this....
2/3rds of the recommended amount goes in the secondary. Then try it out while you're kegging it. If it needs a little more cherry at that time, add it to suit your taste. But my first beer isn't even going to be brewed until later in the week, so don't take my word for it.
