I think you might be conflating two very different types of microcontroller implementations, here. The Propeller is a microcontroller, not a microprocessor like the Netduino series is. As a microcontroller, the Propeller itself is far more powerful than most of Atmel's equivalents -- because it is designed to be.
Arduino is an open source microcontroller platform -- if you were to compare a Propeller with say, an Arduino Uno -- well the :Propeller is running at 80Mhz versus what -- 8, maybe 16? Plus, it has 8 times the cores. It is mature enough to have most of the important components of a system written already -- it is pretty amazing, actually. Plus, they still cost less than $10 in single quantities.
Netduino is a fine implementation of .NET -- that is not part of the basic Arduino platform but, rather an extension of it. I think that there is room to use both, actually -- and I am doing that now.
In my opinion -- right now -- the Intel Galileo and Edison are the best systems available to the hobbyist, but -- I have not used Raspberry Pi much -- although I do own one. The rest -- I have not messed with, but -- Linux based systems are probably where this will all settle down to, eventually.