You have a FMG with a base license which can support upto 10 devices and has a 1GB per day log limit. This is exactly the same as your current FAZ base license.
If 1GB per day is all you need in your organisation then there is no benefit to getting a FAZ.
The problem arises when you want to increase the GB per day limit (more firewalls, more traffic etc).
When this happens the price to upgrade the FMG to increase the limit is quite steep. If you get a FMG +10 device license upgrade then this will increase the limit to 2GB per day. The next jump from this is the +100 device upgrade which will increase it to 5GB per day.
Increasing the GB per day limit on the FAZ is alot less expensive. To put this into perspective, it would be cheaper to upgrade the FAZ to 25GB per day than it will be to upgrade the FMG to 5GB per day.
The FMG with FAZ feature is really designed for companies that want to capture the event logs on their firewalls. If you want to capture more logs for app and user reporting then the FAZ is more cost effective.