This question has come up quite often. The total number of surgeries is important, as well as the frequency, but another factor is the "most recent experience". For example a older neurosurgeon may have done a larger number of AN surgeries during his/her career, but a younger one may be doing a lot of them during the last few years, therefore having more recent experience.
Keep in mind too, that comparing all neurosurgeons to HEI in terms of number of AN removals is not always a fair comparison. HEI does only these types of surgery, whereas most neurosurgeons specializing in skull-base surgery do other tumors too (pituatary, meningiomas etc.). So the frequency of AN surgeries may not be the same, but this doesn't mean that they are not equally skilled.
The criterion I used when deciding upon the neurosurgeon was outcome based. I tracked down posts from here, old patients, the "rate my doctor" website, word of mouth etc. and I chose the doctor who in my view had the best record in terms of outcomes (patient satisfaction, speed of recovery, least complications etc.).
In terms of UPMC, I think there was someone who had surgery there quite recently (I think in the summer), and did quite well. You may want to track them down.