Better to have both. Just be religious in discharging and recharging the batteries of the emergency lights once every 3-4 months. Also there are models that you can charge from a car outlet.

Duracel and eveready now have batteries that have 10 year shelf lives. Check the label. And also avoid units that use "C" size battery as this is hard to find in stores. Mostly they stock only AAA, AA and D.