Sump all the way! If power goes out I dont have any issues with overflow. Two megaflow on each side & pump at the bottom for the return. If power goes off on mines, the water returns to the sump. Power comes back on water does start flowing till the pump kicks back on. I need a new sump as I spec it out better & want two filter socks as the first stage. Trust me you will be glad you went with a sump
PS 55g for me or anything larger will never be an overkilled! You never if you are gonna get a bigger tank in the future & you are set. If you have the room...go for it cause more water vol is a plus!