One main difference (for me) is the fact that subs have rested XP anywhere and FTP or prefered do not. That makes leveling a FTP toon a lot more time consuming that a sub toon. If you don't mind grinding a lot, you may be OK with it.
As FTP you:
- can not use artefact gear
- have less inventory space
- only 2 crew skill slots
- have to pay more planetary comms for gear/mods
- get less money or XP
- just don't give you some quest rewards
If you can live with all those, FTP is actually playable quite well. Of course all those limitations can be overcome by buying unlocks for it with cartel coins, but then you could also sub.