In an extended power outage a battery backup provides oxygenation and circulation so organisms can exchange food and waste. The sump, while not as critical and full of life as the display, is also susceptible to having stagnant water become toxic to the life it contains. A problem that compounds as dying or dead plants and animals release toxins or start to decompose. That toxic water will then be pumped into the display as soon as power comes back. Insert disclaimer here that sumps come in infinite sizes and contents, so the amount of possible exposure/risk also varies. However, the more a sump contains and the longer the power outage, the more it would benefit from having some water movement.
My system has a 75 gallon sump that fills to ~70 gallons of water in a power failure. It also contains a large refugium and frag system. My return pump also runs a venturi skimmer and waterfall algae scrubber. With your criteria of same backup hours of run time, I would absolutely run the return pump over a power head for my system.
Having said that, my battery backup runs a gyre in the display only. The primary reason is because my return pump pulls five times more power than the wave maker. I would have to get a massive RV or golf cart style battery to be able to run the return pump for the same length of time as my current 14 amp hour battery. One change I would make to my system if I did it again would be to run a pair of smaller return pumps. That would provide an additional level of redundancy for a pump failure and I would move my battery backup to one of the return pumps. Then I would have a happy medium between water movement and battery run time. If I had a 40 gallon all in one tank, I would just use a power head backup.
