Done!
I've got both CFS3/OFF and FE with its expansion. They're very different although covering the same era. FE and exp would cost you what, 45-50 Euros or so? I'm sure CFS3 is far less than that and OFF Phase 2 is free (Phase 3 won't be, although I don't think they've announced what they'll charge).
If you like TK's other sims, you know exactly what FE will be. Change the terrain, WWI planes, WWI themed menus, same basic style of campaign/missions. OFF is good for people who liked/didn't mind CFS3's way of doing things. It is sort of a 2-in-1 setup, though. You run a custom front-end UI that then launches the modded CFS3 when you go to fly and exit back out when done. Not perfect, but workable. There's also a bit of oddity where the enemy AI is kinda brain-dead in single missions but very good in campaign. It's a CFS3-related problem, no way to fix it I know of.
I will admit I spend more time in FE, but that's because I fly it coop MP with someone.