I also vote for the C1xx phones. I didn't realize they were still available in such large quantities, which itself is a huge advantage IMHO. But the simpler development model and needed software stack, along with major power advantages seems too good to ignore.
I don't know if the previous discussion/decision regarding NuttX stands. This (limited) RTOS benchmark looked interesting to me:
I am very interested in contributing what I can to such a project.
Scott