As you say, Opterons are mainly meant for servers, so I'd suggest the Athlon. The opteron has a different socket, which means that you'll need a Socket F motherboard, which are usually also server orientated, rather than home user orientated.
Some people still run Opterons because they're often exceptional overclockers though.
But given that you're posting here asking the difference... I'd suggest the Athlon. That's a very nice processor... mine's only a 3800
Edit: Oh yeah, and as for speed... the Opteron has a clock speed of 1.8 GHz, compared to the Athlon's 3.0 GHz... but that doesn't mean that much between different types of processor. You'd have to find benchmarks to get a real idea.
[ 07-06-2007, 10:02 AM: Message edited by: Callum ]