I just re-bought the XL edition. I sold the original last year for $40, amazingly enough. I couldn't believe the used versions held their value for so long so I couldn't help myself, especially when I bought it Day One for $50 (Amazon credit offer). I realized that I needed to have Gran Turismo in my life, especially since I have every other game in the series.
The XL version has so many new things that I haven't even redeemed all my free car tickets yet.
It's an incredible franchise and I love the support they offer for the game. It's like the anti-Madden, where they don't make you buy a new game every year for new features. I love that.