There is a tremendous growth of SmartBoards and their usage in my school district. I am only discussing this topic at the request of one who heard my blurb on a SmartBoard podcast.
Here's the situation, you are on hall duty and you have your warm-up or some other activity on your SmartBoard. As your students walk in (knowing your rules for entering the classroom) someone has placed their fingers on your board causing you grief and anxiety to reload the warm-up or heaven forbid retype/rewrite it.
The Transparent Screen Lock by e-motional.com allows you to lock your screen and still have your PPT or whatever visible to be seen without disturbance! This is the only product that I've searched for that allows this feature (since Smart hasn't figured it out to include it in the Notebook SW). For the full function, it costs about $24....but I believe that it will be a well worth investment.
I guess that would be a benefit of having a Promethean board over a SmartBoard. However, I'm sure those pens will add up if you lose them over time.