I think both Usopp and Sanji have a pretty good chance of unlocking Conqueror's haki but I think it would benefit Usopp a lot more.
Usopp's meant to be the seemingly weak, coward who forces himself to do the right thing when he feels he's truly needed. His whole story is about wanting to become a brave warrior and he already lied about having it so it's pretty likely he'll obtain it. Usopp obtaining haki would act as a sign that he's accomplished, or he's closer to accomplishing, his dream.
I also think Sanji's will is pretty strong in regards to Zeff and upholding all the values Zeff instilled into him. Him obtaining Conqueror's haki would also contrast Sanji with Germa even further by proving that Sanji, for upholding his values regardless of the situation and being willing to die for them, actually has qualities fitting a King, rather than the selfish and thoughtless Germa.
We'll see soon enough. Elbalf seems like a god place for Usopp to unlock his. If Sanji unlocks it, would make sense for it to be by the time his plotline with Germa comes to some sort of conclusion or gets touched upon further.
At the end of the day, I think both might get it and I'd be content even if Sanji didn't show Conqueror's haki. In fact, I'd rather not compare the two and say this one has a better chance since it makes it seem like a competition. They're both characters and I'd much rather look at the implications of Conqueror's haki on both of them.
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Also, Usopp already proved he's willing to die for Luffy, just like plenty of other characters. At Alabasta, he continued fighting against Mr 3 and his female companion, Ms. Something after they mocked Luffy's dream. Him wanting Nami to not say it was just him being hypocritical. He'd gladly die for Luffy, just like Nami and the rest of the crew but he didn't want Nami declaring it as she'd get hurt. Plenty of the crew have declared Luffy will be Pirate King and we've seen most, if not all them, in some way, show they're willing to die for him and Usopp's no different.