I wouldn't compare the VHII to the Demon, which is probably the SD pickup I'm most familar with, unless you figure that they are both meant for super strat style guitars, perhaps as the only pickup.
The VHII is much clearer, a lot more attack, and a more solid bottom end. A lot of people go on about the 'growl' of the Demon, and it is good, esp in a maple bodied guitar, but the pickup sounds like butt in any other kind of body wood.
The VHII has more snarl, more bite, and just more INFORMATION. It's also more dynamic than the Demon. Plus, you can fit it into a wider variety of body woods. My VHII is in an ash Strat and it sounds killer.