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In Philadelphia we have a contract with Promethean. When I asked why we chose Promethean, I was told it was because of the large amount of free online resources that Promethean offers for its boards. (http://www.prometheanplanet.com) The only thing that I really liked about SmartBoards when I went to a session at NECC 09 was the ability to use your hand on the board. From what I have been told, Promethean Boards require a pen (you can get 5 for $100).
Hope that helps!