the 2.8 is a lot more collapsible/compact than the summicron -- not sure why,mostly a matter of more glass. Both, in my experience, are excellent performers, the 2.8 is a bit newer design if ur talking classic lenses, not the new line they came out with a coupla years ago.

Get the best you can afford and take pictures, leave the lens testing to guys with optical benches. In practice, they're both really good.