Deal

In There Will Be Blood, Daniel Plainview chose Union Oil over Standard Oil because Union Oil offered a better deal.

At the beginning of the film, a representative from Standard Oil approaches Plainview and offers to buy an oil lease for a small sum. Plainview declines his offer, feeling insulted by the small amount.

Later he is approached by a Union Oil representative who offers to work with him in drilling and oil refining. Union Oil is offering to pay Plainview a substantial amount upfront and a significant portion of the oil revenues they produce together.

Plainview saw an opportunity to get a better deal with Union Oil and decided to work with them instead of Standard Oil. This decision ultimately led to his success as an oil tycoon, but it also led him down a path of ruthless ambition and moral depravity.