They appear to be similar - because AHK started with old AutoIt source.
"Before AutoHotKey existed, AutoIT v3 was open source. According to the AHK help file, AutoHotKey adapted source code from AutoIT v3 such as Window Spy and the script compiler. When Autohotkey launched, disagreement happened between the creators of AutoIt and Autohotkey because Autohotkey placed advertisements on Google that directly targeted AutoIt v3 searches and displayed Autohotkey as a "sponsored link" ad for Autohotkey app. This included posts on the AutoIt forum that the Autohotkey creator posted with link to his software. Shortly afterwards, a new version of AutoIt was released as closed source."
http://www.wikivs.com/wiki/Autohotkey_vs_AutoIt_v3
AHK:AutoIt = Solidworks:Pro/Engineer
That said, I'd recommend AutoIt over AHK to new users for easier syntax, but if you are used to AHK then that's good too.