The difference in weight is almost irrelevant with an E-bike. More important is power and torque.
Assuming you're sticking to regulation, get one with the most torque, it will feel punchy on acceleration compared to lesser torque models.
Oh and the largest battery it will take-for increased range
What he said.
I have a Cube Agree hybrid, it's a carbon frame. It uses the Fazua motor system. When I weighed it, out of curiosity, with pedals, cages, tubes, tyres, pump and mounts for computer and lights it came out at just under 15kg, size 56.
It has a claimed 50 or 55 nm of torque from its motor, the originals like mine were claimed at 50. The batery/motor pack is detachable, I've weighed it at 3.34kg. The bottom bracket is claimed to weigh 1.3 or .4kg iirc. Peak power is claimed at 250w.
The bike can be ridden without the battery/motor pack, Fazua has a cover available for the hole, but it's not needed. So it would a very heavy road bike 11.5 - 12kg.
The Fazua system is bulkier than a hub motor system so the bikes look chunkier, but some of the newer aero bikes look quite chunky in the down tube, where the Fazua system sits.
I went with the Fazua system because it opened up a range of wheelsets to me. I was a lot heavier when I bought the bike so there was the ability to put an uprated wheelset on the bike until I lost weight. I have not had to go down that route though the Mavic Aksium's seem to be holding round fairly well.
One final thing to consider. Where the assist is coming from. A hub system will put almost no additinal stress/wear on the drivetrain, a mid mount motor will. You may, probably will, experience a higher wear rate on you chain, cassette and chain rings with a mid mount. Keep on top of your drivechain maintenance with a mid mount motor.
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