I think GT5 was a better game in it's form where it had everything accessible, although it's dubious now that they've crippled it by removing online and seasonals.
If you're considering buying it because you want something better than GT6, I wouldn't bother. Buy GT5P for cheap if you can find it. Or get a cheap 360 and Forza 4, because at least that would be a significantly different experience. GT5 and GT6 are mostly exactly the same game.