Which PhD field offers better job prospects: Math or Computer Science?
Assuming that you're interested in the private sector and holding all else equal, I'd guess that a Computer Science degree would tend to outperform a Math degree.  It's just such an incredibly useful field to have expertise in.

That said, if you're going for competitive research positions in overlapping fields, e.g. cryptography, then it may not matter that much so long as you have the appropriate background for that sub-field.  Then, you'll probably be evaluated against other applicants more heavily based on objective qualifications rather than the name that your degree has on it.