Hi Aravs. I was a big HD70 proponent (it was the first sub-$1k 720p dlp) but the mits is brighter, way better out of the box (I normally calibrate for an hour first thing, a week in I've hardly touched my Mits settings, I love the image so much ootb). It's also $100 cheaper in the US. I know it's hard to find in Canada, that might be a factor for you. Also the mits has a big glass lens vs the plastic of the Optoma (between my 6100 and HD1000u I had an Optoma and liked it - the mits feels a lot more professional in the build quality and ease of adjusting the lens zoom/focus). Optoma has the better warranty.
As for the jump in resolution, for non-HD sources that'll basically enable you to sit closer without seeing Screendoor. This is 2.5 or something the number of pixels so it'll look a lot smoother that way, and of course should you go to HD you'll be thrilled! I've found hd-dvd and the mits to be bar none the best video experience I've ever had. The other thing is, the 6100 is a business projector. The mits will not only be brighter. Don't listen to the same brightness spec, once you set brightness/contrast for video the mits will be WAY WAY brighter and enigmatically also have FAR FAR superior contrast. The Benq just happened to be decent for video 3 years ago, the Mits is MADE for theater (as opposed to presentation) and just destroys it in all fathomable picture quality measurements. I'm not subject to the issues of the slow color wheel of the Benq but that's another big advantage of the Mits - faster color wheel, fewer to no rainbows, and the colorwheel is likely a lot smaller and the colors just POP. You won't believe how far the technology has come in a couple years, for the same money you get SO MUCH MORE with the Mits.