The Shepherd Pass route is a maintained trail all the way up to 12,000. I've been up that route 3 times and down it 4. It's a trudge, but an 'easy' trail to the Sierra crest. From there to Williamson, the route finding is straightforward but the terrain is varied. Anvil Camp is a good spot to split the climb. There are plenty of tent sites up at the pass as well, but that location is much more exposed. I've stayed at both locations - neither is better than the other. Depends on conditions and your interests. Higher up puts you closer to Williamson.
From reports I have seen, George Creek is an unpleasant bushwhack and is a less certain option for reaching the summit despite the more direct line.
If you have any flexibility on dates, you should have no trouble securing 2 permits in August for Shepherd Pass during the rolling 2 week 'walk up' permit window. The significant elevation gain seems to reduce demand for these permits. You can expect some other parties on Williamson on a weekend and may have the mountain to yourself during the week.
