Half Dome:
Stats - 4800' of vertical gain, 16.8 miles RT.
That's 570' of gain per mile.
X Factors - Loads of big granite steps. Sandy ground through little Yosemite Valley. Cables. Possibly heat.
Whitney:
Stats - 6400' of vertical gain, 22.1 miles RT.
Thats 579' of gain per mile.
X Factors - ELEVATION SICKNESS. Hard granite pavement or ankle-breaker scralus past Trail Crest.
I'd say Whitney is the more difficult of the two. It's just higher and longer.
If you consider Whitney 10/10 and you did half dome all the way I'd put Half Dome at 6/10 with Whitney.
The altitude makes a huge difference. There is no substitute than getting up over 10,000 feet. I've also noticed that having good nutrition, eating a lot (even when you feel sick)drastically reduces the effects of AMS.
For your training with the loop hike, I'd try to do 2 laps and make it 10 mi with 1600 feet and as you get closer, do even more laps.
