I've seen the Sharp 10k, 12k and a Marantz S2 (in comparison with the 10k). The 10k and the S2 are very evenly matched. I would expect the 12k and S3 to be so as well. Like others here, I'd suggest considering the following:
1) Where do you want to put the projector relative to the screen? Look at the Marantz calculations for screen size and the Sharps. To put the projector closer, you'll be best with the Marantz. If it's further away, Sharp will win out. Personally, now that I can do it, I like my projector way behind me (out of the room, in my case).
2) Price if you happen to be lucky enough that both projectors would fill your screen in your room (there is some overlap, I believe), then price might be a good point. The Sharps generally are more aggressively discounted than Marantz. Sharp is more a "consumer" brand (even in a higher end piece like this) and plays to pricing pressures where as Marantz is generally only sold be higher end A/V dealers, many of which will not negotiate price (or only grudgingly dome down a few percent).
Beyond that, they are very similar and either is going to throw a great picture.
P.S. I think Marantz is doing themselves a disservice by limiting distribution and pricing. Sharp is winning most of these debates because the projectors are pretty much equals, but the Sharp can be had from many more places and for a lot less. If Marantz brought their distribution and pricing in line, I beleive they'd see a lot more sales and the slight (very slight) differences between the projectors would matter more.
