I had 1D3 for a year before, then had the choice between a 5D3 and a 1Ds3 + $1000, took the 1Ds3 and never regretted since.
If you are happy about 1D3 ergonomics (which I do a lot), then 1Ds3 will make you feel right at home, only the VF is a lot bigger, card writing speed faster (UDMA) and image quality better.
Note that my friend has a 5D3, I played with it a lot before I made the decision, and I still chose the 1Ds3, because I value a lot of ergonomic functions in that body that I find missing on the 5D.
0.76x VF, a bit bigger but still noticeable
Projected red LED AF points vs 5D3's black AF points
Ability to bind meter mode to shooting mode, in my case AF linked spot meter (ha!) to M, center weighted to Av
Better Auto ISO, where you limit lower shutter limit and the camera would rise ISO then underexpose to obey it, which I find better for events
Ability to change file quality, card slot, white balance and custom WB in 1/2 button presses
Direct bracketing button
More durable metal mirrorbox / shutter system construction, better weather resistance.
Here's more but I digress, overall to me switching to 5D3 I would solely miss those features. Here's this new saying of "pure photography", I find 1Ds3 to be a pure photography machine. No unnecessary bells and whistles, it does one thing and does it very well: take photos, no bull.