The MITEL, at least the SX200, 200ICP and SX2000 are far and away easier to program because they use intuitive plain english form names (the 200 even displays a menu) whereas with the Nortel you have to be fluent in cryptic LD numbers. This makes for good job security for Nortel techs ;-)
By comparison, the 3300 while still much easier than the Nortel, is very easy to learn if you're brand new to Mitel, but a P.I.T.A. to use for experienced Mitel techs, especially when you need to construct a very large complex database. Example, what it might take me 10~15 minutes to do in the ARS tables of the SX2000 it might take 20~30 mins to do it through the GUI interface of the 3300. This comparison will vary by task, but in every case I find the GUI awkward & slow compared to the CLI of the SX2000.
Mitel offers system admin courses for the SX2K and 3300, both are only a week in duration and go pretty deep, even taking the novice admin into trunk programming and advanced ARS. I think they also have a 3-day admin course for the 200_ICP and also touch on ARS there as well. Anyone know how long the M1 admin course duration is and what they teach?