Well I have been using opt 61 and 81's (all at or around 25.40) with mermail and some callpilot
and callmanager 3.3 thru 4..1.3 with unity 2.x thru 4.0.5
for about 7 years total
started using nortel exclusively the first 2 years approx, then trunked it to a cisco callmanager...within another year we were using cammanager/unity exclusively...
changed jobs and did teh exact same thing...nortel/mermail for the first 2 years...first trunked then migrated to cisco callmanager/unity unifed messaging the last year or 2
as a regular PBX, assuming no call center, i feel its a no brainer/no contest - cisco callmanager and unity all teh way - dopes everything nortel can do and more, all with a user friendly, easy to navigate and get custom with, interface. Custom app's and serfvices can be written and added liek nothing! truly an open architecture where nortel is truly prioprietary and cryptic in copmarison (TDM or VOIP, doesnt matter)
my biggest gripe with nortel, is tech support. THER IS NONE unless you are a vendor..or you need to contract a vendor - they have a useless website too unless you want borderline useless sales manuals (or again unless you are a vendor)
cisco - well theres enough info in their website to perform or troubleshoot any task or issue, otherwise their TAC center and sales reps/product specialist are always willing to assist (yes some of this comes with a cost but well worth it, and you stay hands on in the process)
the ONLY thing i would research in your scenario is the fact that you rely (or will) on call center software. Symposium is tried and tested and a great application. Cisco's call center software and setup has come a LONG way but i dont have enough exposure to it to comment or advocate (or dismiss) its values or its pros/cons. I know of people that love it for the same reasons they love cisco IPtel - VEREY user friendly, very customizable, and you can write or have a programmer write anything possible to enhance its value since its a 100% open architecture system and uses well known and standardized protocols, etc
ideally, you would have a nortel based call center and a cisco callmanager handling everything else like i have had over the years..but the cisco IPCC has been showing up much more in call centers around here in NY, prob for good reason