Details below: following a negative interaction with two NCOs, should I drop it or pursue it further?

You both were wrong. In the beginning, you should have just moved when the Sergeant told you to. The "I'll move when I need to" comment was borderline insubordinate. As for the SSG telling you to move just so he could have the seat all to himself and then talk smack about you with the other NCO....that makes them wrong, as well.

As for taking this higher....As yourself one thing: Is this hill worth fighting for?

If I had asked you to move and double up and you said "I'll move when I need to" in a group of Soldiers I'd have taken it as an inappropriate comment and given you heck too. Probably wouldn't have dropped an F bomb on you and wouldn't have talked crap about you, but you'd have gotten the message that your comment was out of line.
If you came to PSG, 1SG or CSM St.Cyr and told me the story as written I'd ask you what you expected for coming off as a smart butt. Then SSG Numb nuts and I'd have a chat you'd never know about in reference how to make on the spot corrections and appropriate escalation of force verses situation.
Next time just move and when the bus is loaded and seats are empty spread out. No comments required.

Pick your battles SPC. Live to fight another day. Sometimes you're the dog and sometimes you're the hydrant.

You should have moved,as directed. Having you give up your seat was likely because of your initial comment. You've nothing to elevate up the chain other than your own behavior. You've nothing to gain by apologizing, as it's likely over, unless, of course you raise it again.
I'm not signing off on them saying things about you, but you should've done as instructed, not as you felt.
Let it go.

If you told me that you'll move when you need to, I probably would remind you that you need to because I gave you a lawful order in the course of exercising my general military authority. Sure, you could complain about an NCO to the chain of command, but you should be prepared to have to answer for an insubordinate comment. Check Article 91 before you fight that battle. Best of luck.

I think you both demonstrated poor tact. You should be very careful about contradicting someone who outranks you. Unless it's a hill worth fighting on and there may be bigger problems later on if you don't stand your ground, it's not a good idea to say (or even imply) you know better then the person of superior rank. As a Specialist, it's your duty to obey all legal and lawful orders from NCOs and Officers. Unless asked, don't state your opinion when you're given a legitimate order. Even if there were only a handful of Soldiers on the bus, it's still within his prerogative to make you and the rest of his subordinates double up on the bus seats. It would be a bit petty perhaps, but nothing illegitimate to making you load the bus that way.

As for his reaction to your response, that might just be his leadership style of cracking down on possible insubordination. I do agree it was unprofessional of him to trash talk about you with a fellow NCO.

My recommendation is to put this behind you and be more mindful about what you say to your superiors. You have nothing to gain by fighting this.

Bus? You got a bus? http://www.dustyfile.com/the-army-cattle-car was how we travelled in style. :)

Enough said in the other comments. When an E3 and lower the best policy is to keep it zipped unless asking for a teaching moment. E4, E5 is where you learn to grow a military snark, when to use it, how to use it, and how to give a teachable moment to E1-E3s. ;)

Hint: While moving to obey the "order" an E4 *might* get away with "I could have moved if the bus filled up"...

That is not the fight you want to have... Imagine going to your PLT LDR, and he/she asks why you are here, and your statement is that you were disrespected for not moving seats in a bus. Probably not going your way. Better to go to the SGT and tell them you were wrong not to follow his instructions, and that you were not happy that they talked bad about you in public.

You initiated the negative contact, getting shamed a bit seems like a fare trade off. The NCO could have gotten you into trouble for insubordination. Follow the orders of those appointed over you. They don’t have to make sense to you, as long as they are not illegal or immoral, and would not cause the loss of life or limb. Thank you for your service.

Let it go. Pick your battles. I've learned after almost 10 years there are things you don't want to fall on a sword for. Good luck.