Since both are hyper-threaded quad cores here's the basic breakdown, some of which you already know ( and I'm sure mal will correct me if I'm wrong, and I welcome it. )
The 3820 has better ALUs than the 2600K, so arithmetic ( including encryption, ) and heavy number crunching ( including rendering, ) will be substantially improved. It also supports quad-channel memory with faster rates, so memory intensive tasks will be accelerated. If you're constantly throwing heavy tasks at this machine and it's important that it get done as soon as possible, then the 3820 may be the way to go.
However the 3820 is an all-around more expensive platform, and this particular model can't be overclocked ( if that's important to you. ) Not only is the...