Motivations

The Movie: Thorin's desire to retake Erebor is complicated, and many reasons for it are given. Reclaiming his "long-forgotten gold" is mentioned in the singing of Misty Mountains, and when Oin mentions there being signs that the reign of Smaug is near an end, Thorin speaks of the unguarded treasure of Erebor that others may want to take for themselves. However, then Balin argues against the journey, he says that the life Thorin built for the dwarves in the Blue Mountains is worth more than "all the gold in Erebor." Thorin does not disagree, but raises a new point: his father, and his father's father, had dreamt of the day that they might take their homeland back. Thorin appears more concerned with the dwarves retaking their rightful home than he is with Erebor's vast treasures.

Bilbo Baggins is initially against the journey. He is comfortable in his opulent little Hobbit home, and shows no signs of wanting to leave. He not only does not want an adventure, but is shown to be incredibly uncomfortable when something exciting happens at home. When the dwarves insult him by saying he clearly lacks the demeanor to be an adventurer, he agrees with them. Balin's contract, with it's mentions of funeral arrangements and various rhyming ways a dragon can kill, make him faint. Gandalf pressures Bilbo by mentioning his Took side, which had manifested itself when Bilbo was a child and would dream of going on adventures. This still does not convince the hobbit, and he goes to sleep without having changed his mind.

"Oh, yeah, just go ahead and bring up the white trash side of my family. That'll endear me, all right."

The Book: The motivation of Thorin - or any of the rest of the dwarves - is never fully explained beyond their desire for gold. The extended lyrics of Misty Mountains not only mention gold and jewels and harps and gems to the point of excess, but Thorin explicity states that - even though they are "not so badly off" - they mean to get their gold back from Smaug.

Bilbo's motives leap around a little. He first refuses any call to adventure, and squirms very uncomfortably at even being suggested as a fellow conspirator to Thorin's Company. However, his Took side is easily offended, and when Gloin insults his ability to be a burglar, Bilbo angrily retorts that he would "walk from here to the East of East and fight the wild Were-worms in the Last Desert." Bilbo specifically asks about "risks, out-of-pocket expenses, time required, and remuneration." He goes to bed maybe doubting his desire to go on an adventure after all, but does not voice these doubts.

What difference does it make? Bilbo's different approaches to working up the courage/desire to go on an adventure don't matter much at all. The important part of this storyline, for his characterization, is first a refusal to adventure, then an acceptance. Thorin's differing motives are quite significant, however. His keen ambition to reclaim his people's homeland is a marked difference from wanting all his gold and wealth back. 

"Remember when we had more money than God? And now I'm squatting in a fucking Hobbit hole. Man, being poor sucks."

My Opinion: Both of these changes are for the better. Bilbo's characterization is far more consistent in the film; showing him jump back and forth from willing to non-willing would be rather jarring. And I don't like how drastically different his "Took" side is from his "Baggins" side. It almost gives him a split personality, and this series has enough of that shit as it is. Thorin's change gives the character significantly more weight and gravitas. His desire for gold and wealth isn't gone, but it's supplemented by the very realistic, and very powerful, wish to provide a nation for his wandering clan. This makes Thorin not only more sympathetic, but gives real emotional weight to his quest.