The HDs wear like iron and are very good for hard pack and dry conditions, very durable, very difficult to puncture. The GNCCs are made of a softer compound so they wear faster but have awesome traction for general trail riding.
I never tried the HD's but both me and my best ridding buddy had GNCC's last year. We weren't impressed by their forward traction, lots of spinning. We are going some something else this year, dunno what yet.
The GNCC's do wear pretty fast... We're at 50% or so after maybe 3000 miles.
Gncc's all the way, im gonna try the quadcross xc next my buddy has them on his yfz450r. And they do very well in all conditions like the gncc's. The hd's are heavy so you wont accelerate as fast as the gncc's.
Made for more of the mud/wet side of trail riding but still good on hard pack. Wear faster then some but not too bad, light but still very puncture resistant. Best tracking front tire out there imo.