Yes, there are noticeable differences. Take a look at the following screens I took...
SSAO: http://imageshack.us/a/img402/5469/farcry3d3d1120121130185.png ...notice the shadows that appear to outline each of the leaves of every plant making it look "fuzzy"? This same "effect" is also on every object in the world, including NPCs, which causes each one to appear they have a dark glowing outline and the overall scene darker than it should be.
HDAO: http://imageshack.us/a/img713/5469/farcry3d3d1120121130185.png ...notice how everything appears much more crisp and clean? There are no more shadows outlining each object and it actually looks brighter and more natural.
Here's another comparison: http://imageshack.us/a/img545/8343/ssaovshdao.png
If you can't use HDAO, it's better to just turn AO off completely as SSAO has a negative effect on the visuals of this game.
XBL: Tenacious Steve / PSN: Formula409 i7-860 @ 2.8GHz | Radeon HD5870 1GB | 8GB DDR3 | 1.5TB SATA 7200RPM | ASUS MS238 23" LED 1080p