Well, this is My .02cents, I have a redundant sx-2000 system that has been in service for the 2 1/2-3 years, and have Not a had a system wide "outage" caused by the "cpu" cards, and yes it is transparent to the user's when it switch's from "A to B" or "B to A" these are three physical card for each side, I also deal with a Option 11 C system it is Not "fault tolerant" like the Nortel but it wasn't Designed with that in Mind either, There are a Number of ways to "Compair" the systems, but the way I've done it is look at the point's of the proposal, Like How many Nodes it can support, How many buildings, How many digital sets, how many analog sets, What kind of Call Accounting if any, voice Mail Etc, and assign a Number value to each point, 0 or 1 point for something that would not be helpful 9 or 10 for something that would be helpful,
I also do the Move/add/Change for both the Nortel & the Mitel and I like the Mitels interface "Better" but there are numerious sources for manuals for either Mitel or Nortel, But I don't know if you would be doing the MAC work either,
risks, would be something like if the system has Rundunant power Supplys, or if some Aspect of CPU card Failed, What would happen, but alot of it depend on your Particulars,