What can i say: Cisco!!!
In the Miercom page you will find different comparisons between different vendors:
CCM and Mitel 3300.
http://www.miercom.com
I have worked with both Cisco and Mitel (IP 3300,3100, MKC 3050, TDM: SX-200,SX-2000) and I can tell that Mitel are innovative products which were very stable (Unix based), but the main problem was documentation or interaction with the code|protocols handled. It was hard to do advanced troubleshooting like in CCM (CCM|SDL traces, GW debugging etc)
Cisco has tons of docs, books, etc. which makes the understanding of the product easier.
