This probably isn't exact;y what you are looking for - you probably want more technical trade-offs rather than logistics, but since I've just gone through a similar decision, I thought I'd share.
About a year or so ago I bought a 60" plasma - its awesome, and at the time I figured the video piece of my HT set up was well satisfied. Having just bought a projector and screen, clearly i was wrong and while I was in two minds over buying a projector or continuing to use the essentially new Plasma, now that I've seen the 100" screen in action, I also clearly made the right decision in terms of the upgrade to projector. Old hot rodders used to say there's no substitute for cubic inches, and I think the same sentiment applies here - there's no substitute for screen square footage. Regardless of picture quality trade-offs, and that's assuming there are any material trade-offs, a 70" TV cannot adequately replicate the theater-like experience that you get from even a "small-ish" 100" screen, and if you have the space (and equipment) to go bigger, even better.
So the first lesson learned for me that I pass to you, is that if you're looking to create a mostly movie-watching environment and this is destined to be a basically dedicated home theater, make 100% sure 70" is enough and that you get the "theater" feel you want from it, because if you're like me, within a year you'll be looking at a projector anyway.
Other lessons learned from recent experience come down to logistics. Even a 60" TV becomes a beast - its fine if you never plan to move it or want to switch-up furniture, but something that physical size and weight starts to dictate and significantly limit how you use a room, what furniture you can put in it and how you can arrange, or rearrange, that furniture. You end up locked into a single solution because you have this 70" albatross hanging around your proverbial neck. A projector on the other hand? Well most of them are about the size of a small carry-on suitcase and the screens are very mobile, even in fixed designs. It wont take two people and a herniated disk to move it either. It sounds like you have a more permanent room - In my case, I had a dedicated room but we had to convert it back into a "sometimes" guest bedroom. I was able to make it work, but the room became cramped and hard to be in. When I switched to the projector, I saved a lot of space where the TV and its stand used to be. I had a heck of a time finding a place to put the plasma TV though - I had bought it for THAT room and THAT set up, I hadn't considered unexpected changes...like adding a double bed to my Home Theater. ;-) - which is another problem Projectors don't really have. If you move house, need to use the basement for something else, or otherwise want to relocate your HT, your options are very wide with a projector solution, much much less so with a 70" TV. You can adjust a projector across a broad range in terms of screen size and placement in the room thanks to lens shift - you wont be shifting that 70" TV much at all. Projectors also allow you to have a "covert" theater - living room by day, awesome Home theater by night - but the 70" elephant in the room pretty much dictates how that room will look and what you use it for.
Anyways - I can't speak much to the LCD TV versus Projector debate for picture quality, etc, I can tell you that I'm stunned by the quality of my new projector and where I do see a loss relative to the plasma, its 100% immaterial versus the scale and theater experience provided by the projector. In all, I spent $1,800. Granted I went with a manufacturer refurbish projector (full OEM warranty), I got a discount on the screen because the box was a bit dirty - screen itself was fine, and I got the Sony S590 DVD player on a Black Friday sale for $89, but even at full "normal cost" It would have been around $2K.
