i fully agree with lazyplane.
since you have a placement with NTU, why not complete ur degree before making a decision?
not too late to join RSAF as a senior ME (ME4) following ur degree, provided ur certificate of service doesnt look bad.
you may change your mind after serving in the service as a NSF, thus dont lock yourself in unnecessarily for 4 yrs and then regret.

also, RSAF is always looking for AFE(M), due to manpower turnover. thus no need to be too concern with rejecting their offer this time. just tell them that you intend to join back RSAF after you get ur degree, may help you in the future.