If I had to choose between the two, it would be the 300E.
I had a 280E (also a W123) and while it was a great cruiser, it didn't have the same nimble feel of the 300E nor the pickup when passing.
That being said, my 300E has proven to be the most reliable, bullet-proof car I've ever owned. 280,000 Km and going strong on the original engine. Hasn't even had a valve job yet. No oil consumption. Incredible for a car its age.
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired
"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon