The latter is an equally plausible winner, simply because Toy Story 3 is the most beloved nominee in the category. The song itself is kinda derivative, and doesn't come close to the greatness of Randy Newman's previous two songs for the franchise, the heartbreaking "When She Loved Me" and the series-defining "You've Got A Friend In Me". That said, it's still bouncy and enjoyable enough to draw in votes. Being a Best Picture nominee certainly helps.
Or, if none of those options tickle the Academy's fancy, they could stick with the same type of winner they awarded last by voting for "Coming Home" from Country Strong. It's featured prominently enough in the film, but how many members have seen the film? A poor critical response truncated its already tiny release. But if they just listen to the song, that may be enough convince otherwise uninterested voters.
Will win: "We Belong Together", Toy Story 3
Runner-up: "I See The Light", Tangled
Should win: "I See The Light", Tangled
Should have been nominated: "Shine", Waiting for Superman