I am obviously familiar with both MegaSquirt and diyefi.org group
MegaSquirt is a bit different because it's a proprietary product, the major difference with diyefi.org is in the approach: diyefi.org firmware is tightly coupled with the hardware platform they've chosen, and with rusEfi I am trying to be a bit more platform independent.
I believe that with modern processors we can now develop less in "C like assembler" paradigm but more with object-oriented sugar. In the end of the day it's pretty much the same, the difference should be in how flexible and easy to maintain it would be.