I wouldn't turn the Classic down quite so fast... Upgrades are running 4-5 years (2nd contract term). What are they at SIA?
I wouldn't imagine the Classics will be around too much longer, they're supposed to triple in fleet size in the next 4-5 years, so you wouldn't be on it forever.
More important is quality of life, pay, and upgrade opportunity differences at NCA versus SIA. Which one looks better long-term for you, not just qualifications for the next contract you might take.
Good luck to you,