The weight difference between Toyota Mirai [fuel-cell] and Tesla Model S [electric] is 5-20% (the Mirai being lighter).

Not a big difference considering that weight isn't a big issue (wind drag and wheel/road friction are the main factors for range).

For example, someone calculated that simply removing the side-mirrors on a car would increase range by 5+ percent.

https://en.wikipedia.org/wiki/Toyota_Mirai https://en.wikipedia.org/wiki/Tesla_Model_S

First of all, one needs to see how much weight a hydrogen based solution could save at all. With a fuel cell vehicle, you still need a battery to power the car (the fuel cell cannot handle the peak powers, it can only recharge the battery at a constat rate), you need a high pressure container for the hydrogen and of course the fuel cell itself. So this does not leave much weight advantage vs. a pure battery vehicle.

And just a couple of days ago Elon Musk announced that Tesla would introduce battery powered Trucks and Semis even. Seems he thinks that the weight is not a big problem.

Using Aluminum-air batteries as a secondary source in lithium ion cars for longer range would likely be a good alternative. Swap out at your convenience.

https://en.m.wikipedia.org/wiki/Aluminium–air_battery

Weight is practically immaterial, the majority of energy required to drive is in pushing air out of the way, drag.

F =ma. The force you need depends on your acceleration requirements. Going uphill, taking turns, slowing down etc all make mass/weight very material.

Edit: I know weight and mass aren't the same but we are all considering the same gravitational pull so we can disregard the pedantic argument...

I've read that compressed natural gas is a good alternative for trucks.