First, you should check loads on both flights and use EF to check if there is R space available so you can confirm GPU at booking.

Besides that, generally, I'd guess that SFO ICN should have better odds than SFO HKG because the latter is a very heavy route with a lot of feeders and connections.

Not sure what overall route you decided upon but UA has a relatively new SFO TPE route that might be worth looking at. TPE gets you close to MNL and I think Cebu flies out of TPE.

As long as you're looking for R space, consider other options like SFO KIX, SFO NRT, and SFO PVG.

If it were me, the first priority would be getting confirmed R space, then trying to make the rest work after that.

But, if I were only considering ICN and HKG and hoping for a battlefield UG, I'd think ICN is the better bet.