main difference is that the MiNet one is the softphone option (outside of MiCC's own softphone) for call centers. Simply put, if you have ACD you MUST use this MiNet one.
secondly the MiNet does NOT offer point to point video whereas the SIP one does.
The MiNET one is not supported on the MiCollab mobile but the SIP one is and can be used either on the laptop or mobile or both.
There are a few other's but I can't find my notes on them...I'll keep looking