I've owned all three: 123, 126 (SD) and 126 (SDL).

In a nutshell, the 123 is a simpler car and easier to work on. It is also a bit smaller and easier to drive around town. The drivetrain (617) is identical to the SD and both are excellent in terms of durability. They do have a bit of vibration at times and they do make a bit of racket on the highway above 65mph.

The SDL is 6 inches longer than the SD and it is smoother with the 6 cylinder and has more power at highway speeds. Due to its additional weight, I do not find it "peppier" than the SD at lower rpm's and lower speeds. Additionally, it is geared taller thereby resulting in less "get up and go" at lighter throttles.
However, this vehicle comes into its own at 75 mph. It is just unbelievably smooth and quiet at that speed when compared to the 617's. The 617's seem to need to be pushed up above 70 mph and an upgrade at that speed might require maximum available power. The SDL has no such issues. Due to its tremendous length, however, I would not take the SDL shopping around town.