Thats hard to say. Both drivers vont give you 100%. While catalyst supports openCL and some video encoding functionalities, it dont support vdpau and i dont know any software that supports the catalyst de/encoding.
The free drivers do support vdpau for important video-formats but this one dont support openCL.
Without gaming in Mind i would use the opensource radeon drivers, simply because its more save when it comes to kernel/xorg updates. (But if you want to try this, please ask first, because its a little hard to uninstall catalyst on manjaro )
Yes, the old fglrx, aka catalyst driver.
They do and they try hard to get more stuff to the open drivers, wich is a little hard because opensource people only accept “the best of the best of the best, sir”