Technically  this is the wrong Web Site, it is a bit lile going to a  Ford only Dealship and asking a commission based salesman why Ford is a better then a Lambo......

I alway found that depending on who you ask,  will effect the outcome

• Are you asking Voice guys or data guys
• Is your data network Cisco or other
• What is your existing telephony infrastructure

Both systems have their merits, years ago Cisco would say that IPT was cheaper and talk about costs savings - VIOP calls etc as their system had no features when compared to any TDM system so that was their selling point - telco calls are now so cheap however the Cost approach is now dead as any IPT install will always cost you more then upgrading an existing TDM to a IPT, TDM approach

• What do you want to do with telephony - Dial Tone only  or wish to deploy other applications- Cisco i very expensive if DT is the only thing yo want to do
• Is yours firm sale based, field based- Do you need integration into mobile devices such as Blackberry, Mobile devices, Remote working etc  
• How many users does your company have, how many remote offices
• If you have a central organization in international offices how important is Nortel, Cisco support. You would probably get Cisco in most countries as opposed to Nortel
• A few years ago Nortel hooked up with MS and so since MS has the desktop, it looked that that was their way into companies since MS did not have a strong telephony presence while Nortel did but I do not think that has paid off.
• Cisco has links in MS through OCS, the desktop guys like that
• Cisco admin is more GUI based, Nortel is still overlays with separate programs for each area
• Who are your telephony guys - Nortel ?so they will come out probably on the Nortel side
• Who is going to support it - Voice, Data
• Both use linux, less updates
• Cisco uses many servers depending on the apps you deploy
• Cisco can make use of VMware
• Supplier support- Cisco potentially single Vendor support, Account Management, Nortel still separate Vendors to manage?

On a less formal  note, one of the reasons I came out in favour of Cisco when I had to do a review of Nortel, Avaya, Siemens was on the future job market         

That`s my thought anyway , hope they help