I can't answer specifically re a G-1, but I have two G&B jackets in goatskin (one a M-422A) and a Wested Raiders jacket in lambskin. The lambskin is indeed thinner and softer than both goatskins, but what really makes the Wested my "summer" leather jacket is its lack of knits on the sleeves and waist, and its very comfortable cotton lining. I can wear it up to about 70 degrees, whereas my other leather jackets (I've got a horsehide Good Wear A-2 also) top out at around 60, comfort-wise.
But the thin lambskin is, as reported, a lot more fragile than the other hides. I actually tore a big gash in the back panel of the jacket when it got stuck on my car door, which clearly wouldn't have happened with heavier leathers. (Wested replaced the panel with a perfect match FOR FREE, which is some awesome customer service!) Of course, I'm sure lambskin varies just like all leathers, and maybe G-1s are typically made from thicker hides.
What Doctor Strange says about lamb is true; it is easily torn and highly recommend goat. I also recommend that you make the trip to G&B in San Diego. I particularly like G&B's AN-J-3, historic A-2's, G-1's, and Expedition (Indy). Be prepared to leave their factory with more than one jacket.
As others have noted, lambskin tends to be more fragile than goat. Whenever I wear mine I definitely am careful to avoid brushing up against something/snagging it.
I don't think you would notice any difference in jacket warmth if that is what you are concerned about...particularly with both having the mouton collar.
Re; actual weight...I consider my goatskin jackets to be on the "light weight" end of the spectrum compared to my steer and horsehide jackets. Though lighter in weight, it is still a very resilient hide. Get at jacket from a maker that uses high quality goat hide and you will find it very comfortable/soft.
The US Military knew what they were doing when they specified it for their M422's/G1's.