If your applications are multi core capable, then the i7-860 will be the winner.
If your tasks tend to be single or dual core intensive, then the i5-670 would be better.
But... consider the i5-660, it is about $90 cheaper, and only one multiplier slower. I have been testing one for a friend who does scientific calculations, and it is a screamer. It does 2m superpi almost as fast as my overclocked i7-920.