The estimates are dubious...
The only way you'll be able to determine which one works best is to run for a couple of full charges on both and use a battery monitoring tool to examine your active power drain (the amount of power you consume when the screen is on). That's really the only thing that's in play here. I am guessing that between the SMART and normal modes, the SMART is a little bit more aggressive is scaling down the frequency to match the load. So it'll be quicker to get your processor back down to its lowest usable clock rate (i.e. it changes the kernel's governor settings). The idle drain (screen off) difference is most likely negligible.