I think, as in many areas of life, the truth is likely somewhere between the two extremes. As PP points out, the case for Christianity being purely derived from Hellenistic mystery religions has been been overstated by some, and supported by spurious evidence (reminiscent of Hislop's distortions in support of his thesis of tracing Catholic beliefs and customs to pagan origins). And the apologetic argument that there was virtually no input is also unconvincing and strained, especially considering the clear debt of gnosticism to such mystery religions and the complex relationship between second-century proto-orthodox Christianity and second-century incipient gnosticism.
One book I can recommend that critically examines the evidence within the NT writings as well as the supposed external witnesses (such as Tacitus or Josephus) frequently cited by apologists is the Jesus Myth, by GA Wells.