My opinion is that GT5 was overall a better game. GT6 has more content and a more responsive UI, but that's about it. The offline racing GT6 was chase-the-rabbit, the career mode was short leaving much content wasted, car damage was downgraded drastically, B-Spec was an afterthought, the game atmosphere overall was just depressing with its blue hues, and PD for some reason removed the track generator. Performance was worse on GT6 than 5, running at 1080p will surely net you some frame rate loss. The shadows and rain effects were downgraded as well.
GT5 has some negatives, firstly the UCD was badly implemented leaving you scrambling around to try and find the right car, and the XP system was imbalanced often leading to a dreadful end-game grind, the User Interface was unresponsive at times, and there were a few career races that force you to use a standard car.
Overall the experience of GT5 was just more in line with what I'd come to expect of a gran turismo game. GT5 I feel has more replay value due to said imbalanced XP system however and it has a longer career mode. There's also the track generator which offers replay value.