I need advice. I am building a new house with a dedicated media room. I had planned to put a projector in the media room but the wall slopes down limiting the size of any possible screen to less than 100 inches. So..

Should I get the 70" rear projection or a JVC projector with an 80" screen?

Any thoughts would be helpful.

Red

For a dedicated theater I would always do front projection. But that requires that you have complete control over your lighting. If not, a TV is fine. And depending on seating distance, you can go way larger than 80".

Have you considered a 2.35:1 aspect screen? it could be wider and you could do Constant Image Height.
Have you read the CIH chat section here? it is very interesting if you are having a dedicated theater. If this is more a media room then perhaps the Large TV is ok.

What are the dimensions of your room? How far from the screen wall does the roof start to slope downard? Could you possible frame up a false screen wall?

If you don't have light control, go with the big rear projection TV, but more information is needed for a definitive answer.