I had John Tymoczko for biochem in undergrad during the time when he was proofing the new edition. For our class, we used the older edition and the proofs for the new edition simultaneously. They aren't drastically different. Don't buy the new copy if you already own the old copy, or if you don't want to pay through the nose for the new edition. M personal thought on any biochem book is that it should be used to supplement other materials, and I have never, ever read any textbook straight through - any biochem book is bound to be boring!
Let me know if you have other questions.