As the two previous posters alluded to, the answer is not clear in your situation because we don't have enough information.
The 3300 has a distinct capabilities the MiVoice Office 250 doesn't have, the biggest are redundancy, resiliency, and capacity, but these are not it's only strengths... The MiVoice Office 250 is an extremely robust hybrid PBX with it's own strengths.
The 3300, along with a Mitel Border Gateway, are often chosen for deployments like this but the 3300 system is usually centrally located or has 2 resilient locations, to cover a large number of remote locations with minimal hardware investment. It would be uncommon to drop a 3300 controller in each of these several locations.
As to the physical size issue, the two traditional Mitel systems out there now, the MiVoice Office 250 and MiVoice Business (3300) both have 2U chassis, although the MiVB is deeper and full width. Mitel does not have anything as a 1U solution unless you installed a 1U server and a MiVoice Business as a virtual machine.