Avaya has deigned the architecture slightly differently to Nortel in that they encompass all of their PABX functionality and intelligent call routing inside on box. They would tell you this is an advantage of their system
On the other hand (to keep this as objective as possible) Nortel provides a basic ACD functionality within the Meridian that can be roughly compared to the Avaya as follows:
avaya vs nortel
vdn = cdn
asr = esn
vector = doesn't exist but basic call routing achieved in acd queue config.
Nortel has designed their architecture to provide the standard PABX functionality within one box, and intelligent call routing (ccs6) within a server.
Now, without wanting to get into the semantics of which architecture is better (I will leave this to the pre-sales and sales guys!), the one problem I have found with the Avaya design is that when you introduce platforms like Genesys (for call routing) into the equation, you are unable to control treatments within the genesys routing engine (i.e. ringback, silence, busy, music etc), but would rather have to park the call within an IVR.
Obviously this is not an issue if you don't use polish software, but was an issue for us in choosing a switch vendor recently.
Each switch does the job very well, it all depends on what your business requirements are, and what additional solutions you will add to your "voice" network.