tafkam,
The OBi100 is about half the width of an OBi110 and isn't quite as deep or as tall.
While it wasn't released at the same time as as the OBi110, I would bet the OBi100 is the same unit with the FXO port components simply left off. The same firmware runs in both units, so it's unlikely there are any other significant differences.
If your PSTN line will be used solely for FAX, then there may not be an advantage to having it connected to the OBi. If it might ever be used for voice calls, then having it connected to the OBi consolidates all your telephone capabilities through one phone system. The OBi is also capable of bridging calls between its various trunks, so the more paths you have, the more things you might be able to accomplish.