Both systems are excellent systems. The Definity by far has an easier user interface for phone and system admin. The Call Center capabilities of the Definity are superior in comparison to the Nortel. Nortel has a decent call center product (Symposium)but it is an additional server,which requires multiple points of administration.
Where the Definity falls short is it typically costs alot more than a comparable Nortel system. I also find that Avaya creates a dependence on them for their services. Upgrades are usually hugely expensive. Major upgrades are forklift upgrades, Nortel can usually reuse alot of its components.