As I understood it delegation is like loaning - you can claim it back at any time. So there are a few differences.
As you said, STEEM can be traded, also outside of Steemit. Does SP has any meaning outsite Steemit? I don't think so.
When I trade STEEM I trustlessly get the price I want in other currency. If you looking to delegate for return in currency, you have to trust the person you're delegating to. SP doesn't function like a coin and STEEM do. That's the difference.
שום מטבע לא מועבר "מחוץ" לבלוקצ'יין שלו, גם ביטקוין אתה לא מעביר מחוץ לבלוקצ'יין שלו, אתה משלם איתו לאנשים אחרים על הבלוקצ'יין תמורת מטבעות אחרים או שירותים וכו'