On a phone checkout, which “Pay now” button gets more first-try taps?

Version B works better here. The goal is a first-try tap on a phone, held one-handed. Version B's button is large and runs the full width at the bottom of the screen - squarely in the thumb's natural arc. Version A's button is small and parked top-right, the hardest spot on a phone to reach without shuffling your grip.

This is Fitts's law in the open: the time to hit a target falls as the target gets bigger and closer. Pair it with a generous target size and you remove two separate sources of mis-taps at once.

The honest caveat: a full-width primary earns its place at a moment of commitment like paying. Lean on it for every minor action and it stops meaning anything - good judgement is reading the context, not applying one rule everywhere.