Which is inherently sharper? LCD or DLP? The proper answer is almost certainly neither. Each has characteristics though, which may explain why some perceive one type of projector sharper than the other. Then, perhaps more importantly, it's not the type of technology, but the effort and importance sharpness plays when each projector is designed. Some projectors use plastic lenses, some use glass. Most are very good at everything, in that whatever softness seen, isn't likely to harm readability, except, on, perhaps, very small type, type sizes small enough, that many people would be sitting too far away to read it, no matter how sharp.
Above, the Casio XJ-A250V (Casio's newest "Green Slim" projector) is shown handling the highest it can tackle 1920x1080 (HDTV resolution) from a MacBook. You can click to enlarge. Type is definitely softer than lower resolutions, but remember at this resolution, everything is only 2/3 the size of the same data done at 1280x720. In other words, 12 point type appears smaller, the higher the resolution you use.
Tech Tip: DLP's have smaller masks around each pixel, making that mask (the patttern) less visible. LCD's have more visible masks (though the amount of difference between the technologies has been shrinking). It is the slight visibility of the LCD mask, that actually causes many people to perceive LCD projectors as the sharper.
In reality, the real differences will tie first, to resolution. An image on an SVGA projector (800x600) will look softer than on an XGA projector, regardless of which technology is used with each projector. The higher the native resolution, the sharper things should be, overall, but: A projector will be sharpest in its native resolution. Therefore, a slightly higher resolution projector may not appear quite as sharp doing a lower res, as a step lower resolution projector will look projecting native resolution.
That's due to the fact that when you compress text (from a higher resolution) it does get softer. On photos and non-critical imagery, you are far less likely to notice any softness.
If you are going to be doing a lot of work with small type, such as spreadsheets, word documents, etc. it is definitely best to avoid needing compression to work with higher resolution source material. For that same reason, in the sciences, architecture, medicine, and some other specialty areas, there is demand for higher than XGA or WXGA resolution projectors. This is, however a rarity in the K-12 world even though the need for higher resolution projectors at the university level is rather significant.
Bottom line: For normal classroom consideration, sharpness should be a very minor concern. I have mentioned though, the ultra-short throw projectors. Some are better at having the data sharpness consistent across the whole screen. This is a focus issue, made more challenging by the close-in positioning, which makes it harder to have good sharpness from the center to the corners.
Of the two ultra-short throw projectors, the Epson definitely has the sharper image. That said, the Hitachi is sharp enough that the difference will not be a major consideration, especially considering the Epson's an interactive projector and this Hitachi is not. (Hitachi just announced two interactive projectors as this is going to publish.) Sharpness on ultra short throw projectors is something to investigate, or lets say, be aware of. I wouldn't go buying a number of ultra-short throw projectors without checking out the differences.
Projector Contrast and Blacks
I am so used to writing volumes about black level performance when reviewing home theater projectors. In the world of projectors for education, however, great blacks is rarely a concern (for photography), though good blacks are always appreciated.
Generally, DLP's produced the better natural black levels and had higher measured contrast for many years. These days, though, some business and school projectors are using dynamic irises to improve the numbers and the image.
For the most part, the contrast spec is supposed to give you an idea if the blacks are pretty black or just very dark gray, or somewhere in the middle. Projectors with contrast ratios of 300:1 or 500:1 or even 600:1 are on the low end of contrast performance side. Those numbers were typical of a lot of LCD projectors. This year, though, of the eight 3LCD projectors reviewed for this report, all rate 2000:1 contrast, except the Casio (1800:1), the Sanyo (800:1), and a big surprise, the Sony VPL-EX175, which claims the highest contrast in the report, of 4000:1. By comparison, DLP projectors typically start at 2000:1 for these projectors and some claim up to 3700:1 (Acer). All of the DLP projectors did claim at least 2000:1 this year. To get improved contrast though, most of the 3LCD projectors use a dynamic iris. That should be fine, but it's not as good as native contrast in terms of dynamic range.
The bottom line, is that of the projectors reviewed here, on mixed scenes (lots of bright and dark), the blacks will typically prove to be a little blacker with any of the DLP's than with any LCD projectors.
Overall, contrast and blacks won't be an issue except perhaps for teaching a course about photography or graphics. That's very different than home theater projectors, where incredible (dynamic) blacks (oh, say 100,000:1) are very good, but owners want even better. The thing is, in the education or business world one is not looking primarily at very dark scenes. That's when the better blacks make a difference.
If on the other hand, someone is teaching an Art or Graphics class or a better example, History of Cinema, where everyone's watching Citizen Kane, Casablanca, Star Wars and Blade Runner (if you don't know Blade Runner - it's time to! - and I realize that's not likely to be a K-12 class), then a projector with a 2000:1 or better contrast ratio may look dramatically better than some projectors still out there with 500:1 or 1000:1 on some of those very dark scenes. Keep in mind, a doubling of the contrast spec, makes only a small difference. Not one of these projectors, however, can match the dark level performance of all but the least expensive home theater projectors.
For typical classroom or business use, the contrast differences discussed here have to be considered minor, relative to getting the message across.