1. I use Bridge for the metadata tools, simple edits and rating system. 2. To me, it's easier to use than PhotoMechanic. 3. I like how I can append rather than simply replace metadata. 4. My metadata templates are one mouse click away. 5. The metadata tool works better with my Adobe centric workflow. 6. Bridge's metadata is better integrated with Photoshop's File Info. There are items in PhotoMechanic's metadata tool that aren't found in Photoshop's File Info like the Supplemental Categories. PhotoMechanic's IPTC Extension info is limited as compared to Bridge. 7. Bridge seems more polished than PhotoMechanic.
In the end, it's all personal preference. Bridge suits my workflow better than PhotoMechanic which I have tried at home. I will say that PhotoMechanic appears to "work" faster than Bridge. I use an old version of PM at work. I don't know how any updates have affected how well it works.