Project 25 is the Public Safety (Police, Fire, EMS) system. The county also uses Project 25 for Public Works (streets, solid waste, etc) along with the regional jail. The Project 16 is used by the city for Public Works (streets, solid waste, etc) and for Valley Metro and School operations and as a backup for the city Public Safety in case of problems with the Project 25 system.
So if you want the county it is all on P25. If you want Police, Fire, EMS they are primarily on P25.
Basically the Project 16 is the old system for both but the city elected to keep it in place and operational as a backup and for non-public safety users.
John