I think I responded to another one of these iron vs. avl posts, but I'll chip in if y'all don't mind.
I own one of each. The 500 AVL seems to have more power up and down the band, and definitely gets better mileage. It performed well at 14000 feet even, and didn't loose much power....I just had to crank up the idle screw a bit so it wouldn't stall.
To put that in perspective, I tried to light a victory cigar at that height and it wouldn't light.
However, as many experienced people on this site have stated so many times, a Bullet isn't about going as fast as you can, so I'd say they are both great engines, and go for the bike you like best with the right colors, price, options, etc.
Regarding a previous comment about the thump, I've heard an avl with an old style short silencer, and it thumps nicely. I think the main difference is that the older irons (like my 69) have a short silencer, hence it's louder. The 2007 with avl has a silencer that must be twice as long.
I hope to try a twin spark soon. If I do, I'll put a report on the site with my view.