Depends on screen size, most important for lumens is BEST MODE lumens, and how accurate a PJ maintains a NEAR BEST MODE lumens.
The Mits has one of the brightest BEST MODEs for projectors under $1500, the Benq w6000 is brighter for its best mode with more room to spare overall in a NEAR best mode.
Check out projector reviews for more info between different projectors TRUE LUMEN capabilities:http://www.projectorreviews.com/mits...alibration.php
I personally wouldn't want a business projector for HT use, ambient light or not. Several reasons here, one is that eventually you might not always have ambient light (maybe at night) or maybe you changed rooms. The problem with business projectors is that near the same cost as HT projectors, they produce what I would generally consider to be somewhere between a BAD image and a god awful image, but I am pretty picky.
PM me if you need a good place to buy an HP screen, I know a place that sells them dirt cheap.
If fighting ambient light is the BIGGEST concern, then I would get the Benq w6000 and shelf-mount it (it has lens shift) with an HP screen, you'll get a BEST MODE equivalent of 2000 lumens (should be stated in FTL, but too lazy to explain). The insane thing here is you can get a NEAR BEST mode with an HP screen and a BENQ of something ridiculous of the equivalent of like 2500 lumens or more. We're getting brighter than TV's at many screen sizes (depends how big your screen is).
Another alternative to fight ambient light and save some money and stick with the Mits is by using an HP screen and floor mounting the Mits hc4000, with this method you should be able to squeeze a VERY NEAR BEST MODE out of the Mits of the equivalent of 1200+ lumens with BC ON after a semi-tricky calibration to compensate the effects of BC's color shifts and gamma changes. However, the best way to fight ambient light will be to just BLOCK it out, so don't expect miracles with any projector fighting ambient light (it'll still look a tad bit washed out).
Another alternative is to use a few clicks of keystone and shelf mount the MITS upside down with an HP screen, you'll get less gain than a floor mount but still quite a bit of gain.
I have tried a few screens out (but not that many), so far I prefer the HP screen over everything else even given its limitations of viewing angles (which now with the HP 2.4 viewing angles even less noticeable).
I don't see any drawbacks in using the HP screen with the Mits, other than it alters your reflectivity characteristics so if floor mounting or high-shelf / ceiling, you'll need to either have DARK CARPET (low mount) or DARK CEILINGS (high mount), but of course both dark carpet and ceilings are preferred for the best image, as well as no ambient light. Of course if you have some lights on, the color of the carpet and ceilings won't make a huge difference anyways.