I played both when a started at WIS, but eventually dropped my GD teams. Personally, I found HD to be more fun, although I did like GD and had some success at it. What I liked least about GD was that the top teams were even more dominant than the top teams in HD, making it very hard to work your way up to the upper echelon on the game. The fun eventually fades when the same handful of coaches are winning every title.
Regarding time, I found HD to be less time-consuming, mostly because you just have fewer players to deal with and recruit. There also seemed to be more gameplanning stuff to deal with in GD.
All that said, it could be different now since I haven't played GD in a few years.