They have the same powerplant and same transmission AFAIK. Flex has more room for storage, but also weights a bit more. Overall the mods are pretty much identical in potential, and I think you would be happy with either.
I know for the SHO 10-12 and 13+ have some slight differences, but IDK if the same is true of the Flex.
Any major reason you are thinking of switching? I say the biggest factor would be your comfort level driving, your intended use for the vehicle, and maintenance records.