For everyday riding conditions, the obvious place for the battery is the rear rack. It’s the most convenient and logical way to carry extra weight on any bike.
Most experienced bicycle commuters and tourists carry their luggage on the rear rack for the same reason – It just makes sense.
For extreme off-road riding it’s nice to have the weight of the battery more evenly distributed in the downtube of the bike with a lower center of gravity. The Pedego Trail Tracker and Ridge Rider both feature a downtube battery for that style of riding.
But not everyone is a hardcore mountain biker, and the difference is hardly noticeable for average electric bike rider. For casual riding or commuting on typical streets and paths, it’s kind of like splitting hairs.