In my case the answer to why not both was unattended reliability and piece of mind. If it's raining and we lose power we have about four hours before we start to get flooding do to a sump pump not running. Our backup generator will run a self test every week and automatically power the house with a ten second delay when the power goes out. While I suspect solar and a battery backup would keep the sump pump running for a long time it's nice to not need to choose what to run and monitor our power usage during an outage. It's also nice to not worry about power while were on vacation. With solar and a battery backup I'd be concerned about checking it while away from the house. I realize I"m somewhat of a unique situation though in just how quickly we get flooding when power goes out.