I've had the last version of the pre-asph Summicron for about 10 years. It's a great lens.
Is the newer asph summicron better? Yes, from what I've read. If you see yourself shooting a f2 all the time you *may* see some improvement at the corners. But that assumes you are viewing test images made from both lenses side-by-side. By the time you hit F4 both lenses are identical. You may want to see the Erwin Puts review of these lenses at
You are right in that the Summilux is bigger and heavier. Also important is that the Summilux and newer asph are longer than the older 35/2 pre-asph Summicron, which means they extend more into your field of view into the lower right corner of the viewfinder.
Remember, it's easy to fall prey into the belief that if you don't have the most recent version of the lens you won't make good photographs. I catch myself at times doing that.
I have an M2 that is beat to hell (not by me) that I had CLA'd a few years ago. That camera has a 35/2.8 Summaron that is pretty loose in the barrel and looks like it has some cement separation or something surround the edges of the front element. The back element is still pristine. That lens STILL makes images of amazing quality.
My advice, especially when you are trying to keep prices down, is to go for a used pre-asph summicron. If the prices are close go for the asph. I've seen mint asphs going for about a grand, which in Leicadom is a great deal. An M6 with a 35 Summicron is a great package.
If I shot in really low light all the time I would add a 35/1.4 Asph. I would still keep my 35/2 summicron for times I don't need the speed and want the smaller, lighter package.