Bittering hops question

Is there any reason to choose one bittering hop over another besides AA%?
I was under the impression that the flavor and aroma boils off so is there any taste difference after boiling for an hour?

Yes, there are several other compounds in hops other than the alpha acids which give different hops different bittering qualities. So you might say there are hops with smooth bitterness, edgier bitterness, etc.

But you're right that the flavor component is essentially boiled away, so differences in bittering hops are pretty subtle in the final product--you can substitute with confidence as long as you get the AA units correct.

Lots of folks will also tell you to look at the cohumulone level as it is often stated that higher cohumulone hops impart a harsher bitterness while lower levels are smoother.

"It has become accepted dogma among brewers to think of each of these humulones to have different bitter characteristics. There are some that swear that the bitterness associated with cohumulone is "harsher" than that from humulone. Other studies have shown no difference in sensory impact when each of the different humulones are compared. Nevertheless, the humulone:cohumulone ratio is now quoted in hop analyses and new varieties are being bred with low cohumulone levels in mind. Historically, the most highly prized hop varieties - including noble hops such as Hallertau, Tettnang and Saaz - also happen to be those that have low cohumulone levels. "
http://byo.com/departments/884.html

Most people also choose there bittering hops based on the style of beer that they are making.

Just to give you an example, my limited empirical evidence says that Warrior is much smoother than Chinook given the same calculated IBU level. It could have been a number of factors, but I'm looking forward to brewing an IPA with Warrior again.

Beer style is the factor I typically consider when choosing my bittering hops.