Does Sammy quit his job to defend the girls or take a tougher road to success?

You could pick either answer, because both answers can be adequately defended.  Most of the analyses that I have been through over the years regarding this story all like the "Sammy is rebelling against the system" interpretation.  Even my professors in college pushed that interpretation.  I understand that, but I disagree with it too.  

Sammy is definitely part of the "A & P" culture.  He is expected to behave a certain way at work...

You could pick either answer, because both answers can be adequately defended.  Most of the analyses that I have been through over the years regarding this story all like the "Sammy is rebelling against the system" interpretation.  Even my professors in college pushed that interpretation.  I understand that, but I disagree with it too.  

Sammy is definitely part of the "A & P" culture.  He is expected to behave a certain way at work and enforce the rules of the corporation and adult world that he is working in; however, Sammy doesn't see himself as a full part of that world.  That's why he doesn't think highly of Lengel and Stokesie.  When Queenie and her entourage walk into the store, they are in direct defiance of the corporate structure.  They aren't in the proper dress code, they don't walk in the correct aisle direction, etc.  Lengel even calls into question their decency, and Queenie replies that they are decently dressed.  

 "We want you decently dressed when you come in here."

"We are decent," Queenie says suddenly, her lower lip pushing, getting sore now that she remembers her place, a place from which the crowd that runs the A & P must look pretty crummy.  

Sammy envies the way that the girls buck conformity, so his act of quitting is probably partially because he wants to rebel against what the A & P corporate structure symbolizes.  That will definitely give him a tougher road to success.  

I disagree with that theory though, because I just don't believe that Sammy was thinking about all of those things the moment that he quit.  The thoughts may have been there, but I don't think that they reached his conscious brain until after he quit.  I think that Sammy consciously quit in order to impress the girls.  He's 19 years old, and the sight of three girls in bikinis fills his entire brain.  He can't even think straight anymore, which is why he makes a mistake at the register.    

She was a chunky kid, with a good tan and a sweet broad soft-looking can with those two crescents of white just under it, where the sun never seems to hit, at the top of the backs of her legs. I stood there with my hand on a box of HiHo crackers trying to remember if I rang it up or not. I ring it up again and the customer starts giving me hell.

I think that Sammy believed that if he publicly defended the girls, they would be impressed with him.  At the moment of quitting, Sammy probably had more thoughts about a future cute girlfriend than defying corporate structure.