Not a real comparison. Having done a lot on both systems, I can truly say if you want a real IP system...Mitel is the winner in this comparison. The IP in IP Office is a great branding exercise, but the box is coming from a traditional background. Only look to the IP design; Mitel delivers encryption, real DHCP, RSTP, 802.1x, etc all the real IP stuff. ON the telephony site it compares, but look into networking...IP Office max 500 users or 16 locations...what about remote IP phones, Mitel's Teleworker is awsome...LCS integration, the CIS contact center suite, Mobile integration, the phones....
The only downside, IP Office can be offered very cheap, and this is why a lot of customers choose it. Not if you compare the platforms from a technology point of view.