Surely it's not a board choice but a software choice...
It's interesting to see more people discussing the merits of the board rather than the software. Promethean is an education-only company which has bespoke software for the Primary or Secondary environments whereas SMART's notebook is a catch-all application that is focussed more on the Corporate sector.
Having worked for Steljes before joining Promethean my advice would be to standardise on 1 brand so that only 1 piece of software has be rolled out across the whole estate. The biggest single way to get value and a return on your IWB is to invest in training as otherwise they will gather dust.
Having not seen it referenced yet I would urge you to look at www.prometheanplanet.com before making a choice as this is the world's largest and fastest-growing education community for IWB users (250,000+ users compared to SMART's 20,000) and has a huge wealth of free resources to save you time when preparing for lessons and for answering any questions you might have.
Anyway, just my thoughts, hope they help some of you