An IP trunk between two IPO gives you a whole host of features such as blf updates, paging etc etc.
A SIP trunk between 2 IPO will only give you the ability to dial each other, nothing else, no BLF, centralized vm pro etc.
Typically however a SIP trunk is used to connect an IPO to a SIP service provider to provide internet VOIP calls. If two systems are on the same SIP provider, calls using the geographical telephone number are free.
ACA - IP Office Implement
ACS - IP Office Implement
ACE - IP Office Implement
ACI - IP Office Implement