I have wondered the same thing and almost recently posted the exact question. I have no idea on the answer, however I would assume there are only minor differences. In leiu of asking the question I decided I would do some trial and error and find out for myself. I bottled a batch of Honey Wheat last week and added Raspberry extract (store bought McCormicks) to half, and Strawberry (also store bought) to the remainder. Last night I sampled both and was pleasently surprised with the results. While the beer was still noticeably "green" in flavor, both extracts really came through. I even added a little extra Strawberry to my glass for an over the top taste.
I've read a lot of complaints about "artificial" tastes from extracts, which I myself don't mind, or haven't really experienced. I think if you want the true sweetness then the syrups are probably your best bet, although this may lead to a more artificial taste. If you just want to have the aroma and a subtle hint of the flavor, I would think store bought extracts may be better. One thing I did notice while adding more is that it can start to get bitter.
Again I'm no expert, most of these assumptions are just based off my palatte and thoughts. I'm sure some of the experts will come to your aid here shortly, with much more informed opinions.