Durability. You’re paying a lot of money and a costly investment that lasts for a short time is not worthwhile. The electric bike has more components than the common bike, adding the electric motor and the battery; more attention should be put in selection of the bike because of this. You don’t want your electric bike to turn into a regular bike in a short period.
You should also think about how much gear you are going to need to carry. The reason this will affect your decision is because some electric bikes don’t have the capability to mount a front rack while others do. Plan on doing a 3-day trip? You could probably get away with just a rear rack. My advice though would to be to purchase an electric bike with the capability to mount a front rack as well. This way, if you decide to plan a longer trip in the future, you can simply buy a rack and be on your way instead of potentially having to buy a new bike.
E-bikes mostly use motors and battery options from a few major suppliers: Bosch, Yamaha, Shimano, and Brose. A few other brands exist, but are less reliable or powerful. Some, like the Yamaha system, have more torque and others are quieter. But generally all four make good options. Look for motor output (in watts) which will give you an idea of total power. But watt hours (Wh) is perhaps a better figure to use—it takes into account battery output and life to give a truer reflection of power.