Right now I would put Netduino at the top of my big project development platform list for two reasons: 1) the board supports .Net and I have experience with C#; 2) at this time the Arduino form factor (blue connectors in the image below) allows the greatest opportunities for low cost expansion and prototyping. Today these boards are selling for about $35. Unfortunately they are not good for production development of inexpensive products because the .Net micro framework needs a lot of CPU power and memory to run. Here is a migration discussion from 2011. However, high end production products could benefit from this technology. Once again, C# garbage collection is non-deterministic so time critical operations may be tricky at best.
Instructables today. Of particular interest to me is the support for Event/Delegates through C#. I've build many software controllers over the years and I've only used the event driven programming pattern.
Unfortunately I cannot find a Netduino mini or micro single chip breakout, which means this development platform is not yet easily suited for the ultra small, low cost and low power (3v3) controllers without a software/hardware port.
I read one blogger's post that he successfully used the Netduino board with SeedStudio Grove modules. So in addition to roll-your-own shields and components, the Arduino form factor allows a wide variety of component access from a variety of suppliers.