I can only give my opinion. I have an Epson projector feeding a 120" fixed screen in a dedicated theater. I also have a 50" plasma in the family room. Both systems have 5.1 systems with Onkyo receivers feeding full range Klipsch speakers, so sound differences are not the issue.
My opinion is that both systems do a good job at what they do, but they do different jobs. Kind of like asking "should I buy a motorcycle or a minivan?" There is no way that an LED set, even an 80" one is going to give the home theater experience you get with a front projector. It is a very impressive TV, but it is still a TV. That means that ambient lights tend to be on and the family has various activities going on while the set is running. Not a bad thing, but not the theater experience.
When we fire up the front projector, it is totally for the purpose of watching a movie (or sports). The lights get turned off and we concentrate on the show. There is also a big difference between an 80" set and a 120" one. The FP screen has over twice the area, and at 11' seating distance, that is enough to do the full immersion feeling you get when you plunk down money at the local multiplex.
Which would I choose if I could only have one? Probably the plasma, but that is because that room is multi-purpose. If I had a dedicated room, I would choose the front projector.