Is a Nortel or Cisco VoIP certification more valuable to an IT professional?

Your query falls into the category of "good questions" in the sense that the answers to all such questions begin with the same two words -- namely "That depends". In the Cisco versus Nortel or Cisco and Nortel situation, what it depends on is the relative preponderance of one company's technology vis-a-vis the other, and the perceived value of certification in one or both programs at your current place of work, or other outfits that may seek you to job-hop on their behalf.

Purely by the numbers, Cisco certification beats out Nortel by a very wide margin: in a recent survey of permanent VoIP job listings, for example, Cisco is mentioned in slightly over half of all such listings at 51.5%, whereas Nortel gets included in only 8.53% of such listings (about a 6-to-1 ratio). That tells me that Cisco still enjoys a huge name recognition lead in VoIP and other networking certs, but it doesn't tell me anything about how the two companies are perceived in your current workplace, nor does it indicate how the balance of investment falls across those two outfits.

My advice is to look around at other successful VoIP practitioners at your company (and at other similar companies) and to do as much yourself of what they've already done as makes sense to you. I'd certainly recommend at least the CCNA plus one or more of the VoIP specialist certifications (and the CCVP that some of those credentials also require). As far as Nortel goes, if they offer programs on the platforms that your company uses, you could consider going after those once you've acquired the CCNA and CCVP.