I'm a bit bored so I did some tests, AAC at the default bitrate (256 kb/s) vs HE-AAC at the default (64). It's not really a fair comparison and AAC at 256 definitely sounded better. I'm not an audiophile and even I could tell the difference 🙂
When compared with AAC at 128, things may be a bit closer (I didn't test this). HE didn't sound bad (just not as good as AAC at 256), and certainly looks like a good competitor when you have limited space.