I picked out this one post from the forum link that Susan posted. This says it all: -
The two companies have a very different service model and you need to seriously look at them. Avaya is of the old school "rolling trucks" model and will typically send someone out if they cannot fix the system remotely. There is no pleasant way to get a Cisco technician to come out to your site if there is a catastrophic failure of your system (and by the time you do, it will more than likely be your replacement dealing with them).
It's only your mortgage, life, wife, wealth and sanity you gamble if you think of going with cisco!!!
The choice they say is yours.
[Started on Version 3 software 15 years a go]