Bluemagic, I'm afraid your math is wrong here.  While the Bloodlust Brooch is active, it adds 278 AP IN ADDITION TO its base 72 AP.  So the calculation would be:

72 x 120

+

278 x 20

= 14200 combined AP over the two mins.

So, technically, the Bloodlust Brooch still provides more AP. 
Another way to think about it is this, the Bloodlust Brooch (if you constantly activate it as soon as it's cooldown is up) will provide an average of 118.3 AP.  While the Stone will provide a constant 108 AP.

Of course, no one can actually remember to always keep Bloodlust Brooch active 100% of the time it is available, so in actual usage, I think the Stone will still probably do more for your DPS over the course of an evening...