Is it worth is to get a headset with AAC codec for iPhone?
It's hard to say whether it is worth it as it highly subjective.

A headset with AAC codec support will be able to receive audio at a higher quality than the alternative SBC codec at similar bit rates.

The question is whether you are able to hear the difference in sound quality, and if that translates into enough value for you that you're willing to pay a premium for it.

So basically, if you're into high quality audio - go for the AAC support. If you don't care, you're probably not going to notice the difference between AAC and SBS anyway.