They both excel in different areas and comparing the two are like apples and oranges. The Opteron is intended for server use or very intensive applications and will be a more expensive platform due to ECC memory. It will also have limited options if you intend on gaming since I doubt you'll find a PCI-E based server board.
The X2 is a multicore CPU, and will excel in your general desktop tasks. The nice thing about the X2 is the fact that is will handle multiple threaded applications such as video editing or rendering better than most other processors. It'll be the cheaper solution and will yield better performance overall IMO.
They both have their strengths and weaknesses and are intended for different tasks. One is not better than the other until we know your intended use for said processor
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w