I don't think there are any/many comparisons other than manufacturer driven.
I have not played with interactive projectors before. This is something of a concern to me as they certainly offer a serious price advantage. But, I have used Promethian, Smart, and StarBoard interactive white boards.
Frankly, they all are very similar.
The big difference between any of these products comes from the software, not from the hardware. I think this is why interactive projectors are less known and less tested and less used. I'm not aware of any that really stand out with top-shelf software.
Promethean is one of those board manufacturers that does a good job with their software:http://www.prometheanworld.com/us/english/education/products/classroom-software/activinspire/
ActiveInspire software can be used with a number of different interactive projectors, and may be ideal to be used to save some money.
My biggest pet peeve is not this choice you are making, but determining whether a interactive whiteboard makes sense at all, and for what situations it does make sense. After all, what you are talking about is really just a large mouse pad. Someone stands up, like they do in front of a chalk board, and uses it. This isn't always appropriate, and end-users often don't have proper training and the full suite of appropriate software to fully realize the potential of the product.
Then, the PC must still be connected and used for it to be worthwhile at all.
Maybe explain what you are looking for a bit more, and how important it is, and I can give you a bit more information.
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