my recommendation would be to decide if your room is narrow, or short.
if it's short, do CIH since you are limited by the height of the screen. personally, i find that my viewing comfort is based on image height, so if i get a 16:9 screen of the 'right' size, i can go wider at the same height and it just feels more immersive, not more overwhelming.
however, if your room is narrow, you don't want to sacrifice the size of the 16:9 content just to force a CIH set up. CIW is so much easier since all projectors are native 16:9 anyway
in a perfect world, i think CIH is the way to go. it allows me to watch both scope and 16:9 content in 'as big as is comfortable' sizes. but my room being only 12' wide, i opted for a 16:9 screen this time.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC, PS3, XBOX360, Wii
Control: Harmony One