The HC3000u for $375 would have me REAL worried. Again, this is a near $1400 projector, brand new. I'd definitely want to see it in action before making the purchase.
However, if it's in good condition and there's still plenty of lamp life left, I'd still go with it. Yes, a 120" screen is really awesome (I know), but the simple fact is, the larger the screen, the higher resolution you want. Since both of these projectors have the same resolution, the 70" display is going to look better than the 120" display. Especially since you have to sit a lot closer to the screen than I do.
For reference, I have the BenQ MP610 projector. My screen diagonal is 142" @ 16x9 (though viewing 2.35:1 aspect ratio movies gives me a diagonal down to about 115"). My resolution is set to 1280x768. Since I'm sitting roughly 18' away, the image looks fairly nice and I don't notice the "screen door" effect except on static light images/text. Sitting 8' away from my screen, I can definitely notice it and that would seriously detract from the enjoyment of a larger screen.
Screen Door effect sample
The image above is an extreme example of the "screen door" effect, but it does illustrate what you could be looking at with such a large screen, lower resolution, and close viewing distance.
Again, the bigger screen is nice, if you can get a higher resolution, but with your (and my) budget, that's just not going to happen. Go with the smaller screen (after you've seen it in action).