Some of the above I agree with by Joesyah, however a couple corrections:
The Mits hc4000 and hc3800 has a 6-segment RGB-RGB wheel, it does not have a white segment like some of the previous Mitsubishi DLP's did.
My source, Mitsubishi's web site below:
Actually a projector with an IRIS in bright scenes would have no effect 90% of the time as the IRIS opens back up on any scenes above evening time brightness. If it did have an effect, then it would generally be an unwanted effect (usually). That said, that isn't a 100% rule, and all IRIS's work differently, so unless I tested the specific IRIS, I cannot say the exact effects. I have been analyzing IRIS's somewhat and trying to figure out their algorithms, it appears the most sophisticated IRIS's are based on chaotic algorithms of equations of derivatives. A chaotic algorithm doesn't mean exactly what it sounds like it means, it just means that the predicative values of reaction upon derivatives are long and partly theoretical formulas, rather than based on an exact math. That is why IRIS's are all over the place in how they function. Some IRIS's don't even use derivatives much, they use plotting or some much simpler math, and that is why they fail.
The Mits hc4000 is no slouch, but the sp8602 creams it in dark scenes, that's where the difference is, heck that's where the difference almost always is for all more expensive projectors. As far as color saturation and color vividness, I know for sure the sp8602 will have better color in evening-time brightness or darker, but I'm not sure in brighter scenes, and no-one can know that unless dual comparisons are made, they should be very close in bright scenes, but I expect the sp8602 to have slightly better color.
It also wouldn't surprise me if the sp8602 actually had slightly higher contrast in bright scenes, because even bright scene contrast isn't 100% translated from ANSI, and the 8602 is likely to maintain a very high ANSI just like the Mits but also the sp8602 will have a greater contrast across the range of scenes, in that sense it is possible to get ahead of the Mits in bright scenes a little, but it would not be night and day.