From my experience, both Werner and Bigshot are correct ......
Some people can discern 320 mp3 / 256aac from lossless - but my experience so far is that there are the ones that say they can, and the ones that prove it by showing dbt results (eg Foobar dbt summaries). I know a couple of people who are trained listeners and have shown that they can - but even they admit that it's often very difficult, and generally not the "night and day" that is often suggested (particularly by people who have not formally conducted tests). If Werner can tell the difference - I have no reason to doubt him - especially if he's tested himself. Some can. Most can't (assuming the tracks are both ripped properly from same source, and encoded properly). There is no stigma if you can't.
FTR - I tested extensively, and I can't tell the difference between 256aac and flac. For me - that's actually a positive - more room on my portable.
OP - if you follow Werner's post (immediately following your initial post) it'll save you a lot of space. I would recommend ripping to lossless (for archiving / desktop playback), and converting for portable. The reason is simple - if you rip to lossless, you can then transcode to any bitrate you wnat, and you will nto have to re-rip your CD. EG - I rip to flac for my desktop (I have the space, so no reason not to), I transcode to 256aac for my iP Touch 4 / iPhone 4, and for my two kids (they have smaller players) - I convert from lossless to 192 vbr.
If you want to test yourself (PC) - set-up audio player software called Foobar2000. Install the comparator plugin. Rip a CD you know really well into FLAC, 256 aac, 320 mp3, 192 aac. You can then double blind test yourself. It's worth doing - simply for the fact that you'll remove any doubts you have now.
But to your original question - for portable - 256aac should be pretty much indistinguishable, and will save you a lot of space :)