I know I’m a bit late here but for posterity sake, the reason why the 18ss is more expensive is due to the “collectivness” and rarity of it (more especially now). The T3.0 is newer and out performs the 18ss however.
That being said, I’m a fan of all of HPI’s engines. I compare them to the stability of a Toyota. If you know how to tune and take care of them, they can last forever.