I've listened to both (because I have to learn one for an audition), and to me it sounds like D is a bit more challenging. What's your opinion? Also, which one do you personally like better?
“One good thing about music, when it hits - you feel no pain.”
-Bob Marley
It depends. G is usually considered to be more technical, but D is considered to have more challenges in regard to phrasing. Or at least... that is my opinion from when I learned both of them. Either way, both are very well known, and have to be played spotlessly.