I've given this one some thought too
I have 20/20 vision and at 11.75' the only difference in resolution that I can really make out between the two sizes would be the text in blue ray menus or title screens. The characters at 108" look the slightest bit jagged whereas on the 100" they are perfectly smooth, moving about 2 ft closer from my sitting position on the 100" screen also reveals the jaggedness so its not a projector issue.
One thing I haven't done is look at some DVD content... I have an oppo 103 and hope the video processor will do a decent job. I figure there won't be much of a difference between the 108" and 100" with SD material at my distance.