hmm. that's a tough one. Really similar bikes.
I have no experience with the lightning.
Here's what's good about the Giro 20...
flexibility. You can lay the seat WAY back. You can put rim brakes or disk brakes on it.
you can run a 700c on the back. you can put gigantic tires on it.
you can get super fast light wheels and tires.
you can swap out the seats. there are 3 different styles available.
you can change the handle bars from b-pivot to a fixed riser.
handle bars can be changed from the wide to the narrow ones.
I put a carbon seat on mine and a 24 inch front wheel. It was a fantastic bike.
comes in steel or aluminum
There's a potential to build the aluminum framed one down under 24lbs if you bought Ultegra components and used a carbon seat, stiff riser, narrow bars, and built a wheelset with aerohead rim and light weight hubs.