FusionPBX vs. FreePBX
FusionPBX has Extra Features Included
FreePBX comes with support for many 3rd party phones, but it's an extra cost. As is multiple parking lots, class of service, and a few other features. FusionPBX includes those by default.
FreePBX also has high availability, but it's also an extra cost and requires the backup system to be in the same building as the primary. FusionPBX's failover doesn't have to be in the same building, city, or even country. It can also be strengthened by adding more systems to the cluster to keep your system running.
Open Source vs. Open Source*
That asterisk is important. FreePBX's core system is, in fact, open source, but it locks additional features behind proprietary, commercial modules. That's a valid way to pay for an open source project.
However, Sangoma pushes a rhetoric that no one can sell modules. And while proprietary modules can't be built by companies other than Sangoma, anyone can build one to sell. Sangoma actively fights against forks and tries to prohibit modules that replicate features similar to its own modules.
FusionPBX develops in the open and has clear guidelines for contributing. It also includes many of the same features as FreePBX, but it included in the open source portion of the PBX.