The difference is only 50 to 100 lumens between the 2 projectors in best mode, this is not even visible by the naked eye and whichever one comes out brighter in the end will be related to how perfectly you calibrate. The Mits looks better OOTB than the Optoma hd20, it doesn't need a calibration nearly as bad, and the brightness difference is not enough to even be a factor in this decision.
The hd20 only has 1,200:1 On/Off compared to the Mits hc4000 which has 3,800:1. The production version of the HD20 may be a bit brighter than the reviews show, not sure as you'd have to go into the hd20 thread to find that out, but I have seen no immediate evidence of it.
The two projectors are too close in brightness for this to even be a deciding factor. Anyhow, I'd buy the Viewsonic Pro8200 before the Optoma hd20 if you want brightness, even the Viewsonic has around 1,500 to 2,000:1 On/off (depending on calibration and placement) and Dark Chip 3, better color wheel, and brighter than the HD20 for a lower cost.
From Projector Reviews (Projector Central also found the same Lumens)http://www.projectorreviews.com/opto...erformance.php
The dissapointment comes from the Optoma claim of 1700 lumens. Now, most companies tend to overstate their brightness, and often significantly. That said, let's say that Optoma may have gotten a bit carried away, since it measured almost 40% less than their claim. (15% - 25% isn't that unusual, and it's rather uncommon to find a projector that beats its marketing claim).