FreeRTOS does not have good support on IAR and generate many warning(20) when compiling code,I can not modify the source code to remove the warnings,Do you have modified version?
UCOS is RAM consuming(not good for AVR limited RAM).
I tested only ucos on atmega 128. Possible to use for small tasks without external RAM (i meant atmega128) otherwise - external ram is mandatory. Yes, it is RAM consuming but no worry about migration to another platform , let say arm or MC32 or almost any other mcu.
Better to choose it if you are not planning to use avr forever. )