I thought ahci emulated was the same as IDE but it emulated SATA so that the devices could communicate. So if you are using ahci you are not going to be able to take advantage of SATA speeds or hot swappable capabilities because it's just going to run at the 133 speed. That is why you ALWAYS use SATA unless you are on a SATA drive running a system older than Vista (IE XP). Because XP doesn't have the SATA drivers so you have to use ahci which emulates the SATA drivers but is essentially using IDE technology.
That was my understanding of it - I could def be wrong though.