Should I buy a Netduino or Arduino?
I love C# and find the productivity benefits of developing in C# with Visual Studio to be a very strong factor in favor of the Netduino.  I would not pooh-pooh Netduino because "C# isn't for microcontroller/embedded environments" as other answers here indicate.

You can make great fun projects and learn a lot about connecting different physical inputs and output devices using Netduino just as you can with Arduino or going "bare metal" and doing the direct MCU management of the AVR, PIC, or other microcontrollers.  

With that said I am sticking with Arduino for the following reasons:


Better community support.  Arduino just has way more samples, examples, and a bigger community you can draw upon.
Netduino doesn't have a story for going beyond the expensive kit/prototyping approach.  Going to implement an interactive art exhibit requiring dozens of microcontrollers running your code?  Get ready to shell out $35 each for the Netduinos.  With Arduino you can prototype on the Arduino and then implement on a bare-bones MCU-only solution with the AVR microcontroller for under $10 each.


So I would say if your C skills are dusty or nonexistant,  you love C# and Visual Studio, and just want to tinker on a few hobby projects that won't need to scale out to where cost becomes a bigger issue, go Netduino.

If you are comfortable in C, or comfortable in getting by in it with the help of code samples and the community, and want to be closer to the metal and look at eventually getting away from the "prototype" approach of Netduino and Arduino, go Arduino to get started.