I'd go for the Super Adjustable, personally. The black handled Super Speed is quite possibly my least favorite of the entire Super Speed line (they don't feel at all substantial enough for me -- very light and somewhat flimsy feeling). If you've never held either, I'd recommend getting your hands on a couple before you go hunting for a nice specimen birth razor.
The Slim has a 3 1/4" handle whereas the SA has a 4" handle. Mechanically they're both similar. However, I find that the SA has better balance. I'll get a much more comfortable shave out of my SA than I do from my Slim because of it.
I'm in the same boat as the OP, my choices are the same two razors. The Super Speed is a tinny little turd compared to earlier Super Speeds that are far more robust and well made. My birth year razor ended up settling on the short handle T3. Better feel and balance than the long handled version, and same door profile as the highly regarded slim adjustable.
If you are shopping for a birth year razor, then ultimately you need to get every razor that Gillette made during your birth year. So you need at least the Super Adjustable, Superspeed, and a metal Atra handle. There are also 3 piece techs available, but not made in USA (I believe).
I'm a 1976 and I have bothe the SS and the SA. I wish the Atra was made in 76. I prefer the SS and it is my goto travel razor.