Simon Telephonics costs $6, does not require an OBi ATA, adds Caller ID name, supports more codecs, supports TCP/TLS, and offers fax-to-email.
The third-party firmware is free, and makes your OBi work exactly as it always has. It does not require the Simon Telephonics service.
In terms of making and receiving calls, both methods do more or less the same thing. Start with the free option, and if you decide you want the extra features of Simon Telephonics, you can always upgrade later.