How is Sid different from Tom?

Sid is a great foil for Tom in the book. Unlike Tom, Sid is quiet and mindful of his manners. The text tells us that he has no "adventurous, trouble-some ways."
In short, Sid is the total opposite of the mischievous and rambunctious Tom. In the story, Sid exposes Tom for going swimming, and Tom promises to get his revenge.
Of course, Aunt Polly prefers Sid to Tom. Upset that Tom is always misbehaving and getting the better of her, she orders him to whitewash the fence. You can read all about it in chapter 2. For his part, Tom isn't about to whitewash the fence if he can get someone else to do it.
In the same chapter, you'll read about how Tom manages to trick other boys into doing his work. And that's not all: Tom even accepts a reward (an apple) from Aunt Polly for his supposed industry. On his way out, he manages to steal a doughnut from right under Aunt Polly's nose. The poor woman never discovers the truth about Tom whitewashing the fence.
Basically, Tom managed to manipulate other boys into doing his work by portraying whitewashing as an enviable task, fit only for the best boys. In other words, Tom portrayed whitewashing the fence as an exclusive activity.
Tom's wily ways present a total contrast between him and Sid. On the way out, Tom throws a hail of mud clods at Sid, who is caught off guard. To Tom, this is a satisfactory way of paying Sid back "for calling attention to his black thread and getting him into trouble."
Sid's good behavior may put him on good terms with adults, but his smug attitude often puts him at odds with Tom. Both Tom and Sid are total opposites. The former is mischievous and thrill-seeking, while the latter is virtuous to a fault.

Sid really couldn't be more different to Tom if he tried. For one thing, he's a real goody-two-shoes. You'd never catch Sid playing hooky to go swimming, or getting into fights, or running off to play pirates and pretending to be dead. The boy has no sense of adventure. He always does as he's told and never gives Aunt Polly the slightest trouble.
It's no surprise that Sid doesn't get on with Tom, given their huge differences in personality. Sid resents Tom for his waywardness and lack of discipline, and he is always looking for ways to get Tom into trouble. Sid is a real sneak and has no hesitation in telling on Tom to Aunt Polly. We see this when Sid observes that the thread on Tom's collar is a different color to what it was when he set out for school that morning. This alerts Aunt Polly to the fact that Tom has been playing hooky to go swimming and has fixed his collar with a different color thread.