Hawk,
If you want performance... stick with 2-1.... no question there.
There are really good dual systems out there, but you need to stick with a 2-1-2 setup if you go that way. The Power Duals head pipe by V/H is great and is better that true duals 2-2 ( speaking from a performance perspective ) Kuryakyn makes a similar X pipe design for less $$$.
It's not all simple, because of all the different design aspects of each brand... but
If you want looks... go with duals in any configuration you choose.... sound will be key in this scenario.
If want more performance with duals... get an X Pipe header setup...
If you want the most power and torque you can get.... go with a 2-1 ( even with a falsee for the dual look )
I like the Thunderheader sound and performance gains for sure... I just can't get past the look of it... I just don't like it at all.... I have the V/H Pro Pipe that replaced a de-cat header and Rhinehart slip-ons. My gains were definately felt across the board... from throttle response and torque to the high rpm range and roll-ons. Of course the new map I flashed with my FP3 helped with this for sure.