One as to ask just what camera are you using now. You dont own a 5D2 and your printing out photos that a 7D can't handle at just 18MP........ wow
also in another post about the 7D you say (I think its at the limit of lens resolving power with 18mp, so there likely wouldn't be an increase in resolution) so what lens are you GOING to be using on a 30 MP 5D mk3
I only have one camera at the moment. I would own a 5DII, but I was holding out for a 5DIII hoping it would up the resolution and improve the AF (which is truly atrocious for a professional-grade camera, used for landscapes or not.)
Regarding resolution, you forget that the 7D is an APS-C cropped sensor, while the 5D is a FF sensor. Assuming the two formats had identical resolution, a FF 35mm size sensor would need 46.7mp to reach the same pixel density as the current 18mp 7D. You can see the math in my answer here:http://www.canonrumors.com/forum/index.php/topic,2319.msg49501.html#msg49501
So, assuming the 5D III was 30mp, I wouldn't sweat a drop...there would still be 16.7mp of headroom before you outresolve the lens. Additionally, I'm less concerned about the diffraction limited aperture where diffraction *starts* affecting sharpness...and more concerned about where sharpness is visibly degraded...which is usually several stops later. See my answer to the following question for a visual example of how optical aberrations wide open have a far greater impact on sharpness than diffraction at much tighter apertures:http://photo.stackexchange.com/a/8339/124
Regarding print...I like my work to be huge.
When I can afford it, I prefer to print at 55" x 36", or about 4'5" x 3' (my home has a surprising amount of expansive walls in every room and down every hall that need something large to fill out. My living room wall, which is still empty, could really use something more along the lines of a 6'x4' print.) At native resolution without scaling, the 21.1mp sensor of the 5D II, which produces images 5616x3744 pixels in size, can produce a print 18.75" x 12.5" @ 300ppi in size...smaller than my preferred size three-fold (i.e. I could fit 9 whole 18x12 prints in one 55x36 print)!
If I had a 46.7mp sensor at my disposal, that would be an image size of 12430x8286 pixels in size. Thats a native print size of 41"x28" @300ppi! Much closer to my preferred print size (about 75% there), and close to the print size many of my favorite landscape photographers (who tend to use 4x5 Velvia 50 LF film printed at around 50"x40"). So, as far as I am concerned, so long as we are not out-resolving lenses and diffracting our way to fuzzieness, more resolution can only be useful, not bad. Going from 21.1mp to 18mp is a step backwards, when the physics and the math tell us that were not even close to the limit yet, and advancements in sensor fabrication keep lowering noise, improving color depth, and enhancing dynamic range all while still increasing resolution.