The opty is built with higher end silicon so it overclocks more, and the X2's have a 128kb L1 cache, but im pretty sure it dosent make a difference.
the opty's originated in servers which put more stress on the CPU, so AMd has put better more durable silicon on the CPu so it can handle more stress but it can overclock like a beast, some people have gotten an opteron 146/165 (same stock speed) around 3.0ghz!