The Opteron dual cores, 165, 170, and 175 all have 2x1MB of L2 Cache, while the X2s all have 2x512KB of L2 Cache, except for the X2 4400+ and X2 4800+ having 2x1MB of cache. The Opterons also run at a lower stock vcore, which means more headroom and lower temps. The Opterons have been seen to be able to OC better then the X2s, most Opterons go up to around 2.7-2.9ghz, while X2s top out at 2.6-2.7ghz.
If you can afford the dual core, the Opteron 165 starts at around $300 or so, then definitely go for it. The X2 3800+ is around $280, so definitely opt for the 165.