it's as simple as grad - pay and promotion prospects good and fast, nongrad - pay and promotion prospects not so good (initially!)
Yes, you have that correct - payscales are fixed to rank, and rank progression is time-based to Flt Lt - non-negotiable (unless you f**k-up, and can then be 'negotiated' back a year or so!)
Apologies for misreading your intentions, and the best of luck for IOT.