It's very difficult to advise anyone on the "perceived" quality of a particular setting. People will have different preferences and be using different hardware for playback so it's always best to go by your own ears. Choose a CD track or tracks you are familiar with and rip two copies into iTunes and have a listen for yourself. All you can really say about the two settings is that iTunes Plus (AAC 256 kbps) will produce larger files so if hard drive space is an issue for you then you might want to use High Quality (AAC 128 kbps) for that reason alone.
You could also have a look at this article, you'll get an extensive comparison of the AAC and MP3 formats and bit rates benchmarked against CD format:
Planet of Sound - Quality of AAC audio and MP3
