The Onion Valley / Kearsarge entry is definitely an easier hike, since you start at 9k elevation rather than 6.3k. It is an easy cross-country walk (~2.5 miles) east-south-east off the JMT to join the trail from Shepherd Pass. (But then with the Kearsarge and Forrester passes, it's about the same elevation gain as Shepherd Pass alone.)
The Shepherd Pass trailhead requires a high-clearance (if not 4wd) vehicle.
As for AMS, when I hiked from Onion Valley to Whitney, I had AMS until the third day, and we even spent the night before at OV. Climbing the Shepherd Pass route is just a suffer-fest that will certainly get you into AMS-ville sooner.
The most difficult part of this trip would be getting the Trail Crest Exit permit along with the entry permit. Alternately, you could exit via the Mountaineer's Route on Whitney, or via Horseshoe Meadows.
