Wex Photographic have posted a really useful table showing the differences between the Canon 5D Mk III and the new 5DS.  Click the image or link below to see it.

http://www.wexphotographic.com/blog/canon-eos-5ds-vs-5d-mark-iii-22-key-differences?cm_mmc=Facebook-_-Wexfacebook-_-blog-_-canon-eos-5ds-vs-5d-mark-iii-22-key-differences

Having looked at the table – I’m quite happy to stick with my 5D Mk III – For me the older model wins out on a number of key areas – ISO, frames per second and battery life, not to mention the fact that you cans capture more images largely due to the fact that they are not silly Megapixels in size.

Yes, there are some nice improvements, such as the built in intervalometer, the fact that the autofocus points will now light up in AI servo mode.  But these are minor not worth the price tag.