Netduino was cheaper than FEZ - until FEZ came out with Panda. I recently purchased a Netduino and a Panda. Both are good environments with solid support and a good forum.
My take is FEZ is more mature but I also like the netduino community as well - so not taking anything away from netduino.
I was able to port over code pretty easily (small apps). If you need raw pin speed - neither one is good. Outside the pitifully slow pin write speeds - they seem like a nice alternative if you need more memory than a mega provides.
Personally .. I am going to buy go with a mega and see if I get over my arduino limits that way before I continue down the MS path.
That said .. I can say in under a week, the move into this environment was not bad and in some cases is was "freegin easy". If you are working with standard SPI and the like, you may not hit the same speed issues I did.
Pretty interesting movement - jump in if it feels right - It won't hurt 