I have not compared the texts synoptically. Yet, since the digest is a more comprehensive, I think it’s more an update than an expansion. That being said, the updates can be seen as marginal – a random table here, an advancement mechanic there. Yet, they’re worth to be called the Core Rulebook, 2nd edition, framed and styled differently for TOS.
Regarding the new advancement rules (which I, personally, think are enough reason to buy at least the Digest’s PDF), you might want to look at this thread on Character Advancement, this thread on Character Arcs, Klingon Style and finally this thread on the New (Klingon) Reputation System. It might give you an idea.
Both the new Advancement Rules and the new Reputation System (that fuses in the Reputation based Awards & Commendations system presented in the Command Division Supplement) are great. For me, they’re enough to buy either the Klingon Core Rulebook or the Rules Digest. Your mileage may vary, of course.