HiFi works well for me. If I had the money, I would definitely go premium since there is a noticeable difference, but not enough to be a dealbreaker (the added EQ functionality is very nice vs. the base systems treble/bass adjustment).
IMO, it depends on what type of music you listen too. The premium system would really bring out classical music better like mozart or something. If you trendy music like snoop dogg or ke$ha (w/e those teenagers play on the radio today
) then HiFi should be good enough. The premium would make a difference when listening to that kind of music, but its nothing to write home about
If your looking for the "out of this world sound difference compared to stock" or the "wake-up-your-neighbors-with-booming-bass" kind of sound system, your best bet is aftermarket.
btw, I have a 2010. no clue if its the same as a 2011 or 2011.5, but I doubt it'll be that different..
