Gold includes expansions and the option of buying further expansions. HD is the vanilla game but with revamped graphics, soundtracks and UI.
Which one is the best option?
Both are great. I found HD to be so much better looking, that it made the game much more fun and immersive for me. I'm not sure I'll ever have time to play the many expansions I got with Gold. The "vanilla" HD is plenty large. I started over with the HD. Loving it