Hi Amin, welcome to the Cameralabs forums!
The FZ50 has several things the FZ8 doesn't have, including slightly higher resolution, a flash hotshoe, a lens barrel which doesn't extend, a flip out screen, manually operated zoom. If you really value these things, then go for the FZ50. But if you can live without them, the FZ8 is remarkable value at almost half the price.
You might want to have a look at the verdict of our Sony H9 review as it compares several superzooms: