how big do you plan on printing that you find 10MP to be lacking? don't forget that you are going to need tons of storage for the 1DsmkIII because the files are going to be HUGE!!
pretty sure that file sizes jpg are based on O( N*log(N)) rather than N^2, so 1dsmkiii files will likely be only partially larger than 1dmkiii (maybe 6mb instead of 4) and raw files are linear with respect to MP given the same bit depth, so it'll be just about twice the size.... 25mb raw is nothing to laugh at, but it's not the end of the world either (but you will need cards and hdd space to make up for it)


