555SL is somewhat lighter then 501, and is just a bit heavier then 531.

Personally, I'll grab for a 555SL machine just as soon as I'd grab a 531 machine. In my book, they're equal enough - less then a pound difference between the two won't hurt matters much.

P.S.: I own two 555SL machines ('84 Competition, '85 Super Course) and enjoy them both. My favorite is my '86 Grand Prix though. It's 531 main-frame tubing though, according to the label - they made the switch in '86. I believe the rest is Tange, same as the 555SL machines.

Take care,

-Kurt