I used both vacuum sealer before, and I hope my experience may help.
Manual sealers give you more control when sealing certain soft foods you don't want to get crushed like berries or bread. Alternatively, liquids that might spill over into the machine. (An automatic sealer's mmanual mode have to be utilized for liquids anyway.)
Manual models can be less finicky then their automatic counterparts. There exist fewer chances for bugs, including the sensor not detecting the bag. Manual models also require less bag material, since the automatic models require that you insert more of the bag material on the machine in order for the sensor to detect it and trigger the sealing process. That excess material gets wasted.
All in all, I prefer automatic one, for its sealing ability is more potent. FYI, I am using a crazysales
vacuum sealer, for its low price and high quality, and as mentioned, automatic performance.