I think there are even more tastes: the one you call mongodb-csharp is actually two:
The first is a little more mature and is widely used in this area. The second is recent development, but it comes from 10gen, the creators of mongodb. The implementation looks good and rather looks like a samus driver. If you now need to do something in production, I'm not sure what to advise you, but in the end I will go for the 10gen driver.
The only thing that does not currently offer is Linq integration. This may be important to you.
I have no experience with NORM and simple-mongdb drivers.
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