Fax over voip is 2% knowledge 98% voodoo and unicorn tears. Honestly it typically "just works" or "never will" Your best bet may be to go physical line as mentioned above or the Faxxbochs route. If all cost are truly equal it probably doesn't matter which.
Shameless plug:
Faxxbochs is a Sangoma company which is also the corporate sponsor of FreePBX. So I would encourage this route if all cost being equal.