Well, I voted for Quinn before I read your other son was Sebastian. I want to change my vote! Both my dd's have 9 letter names, and they do complain sometimes because they are long. I couldn't imagine how they would fight if one had a long name and the other didn't. (You will find sibs fight over who has more square footage in their bedroom, who got the bigger cookie, etc. Yesterday they were fighting over who could take the plastic cover off my new mop! Sheesh!) They do love each other, though
Think of how the names will sound together as you're shouting them across a crowded playgroud, "Sebastian, Alexander, it's time to go!" or "Sebastian, Quinn, it's time for supper!" To me, Alexander flows better. I also have a friend with a son called Xander, and I like that, but unless you have a nickname for Sebastian, I'd definitely go with Alexander.
Also, don't underestimate the "QuAC" factor. You don't want your son to end up being called "Ducky!" (And that Mighty Quinn song is now stuck in my head from thinking about this

)
I hope you have a beautiful heathy son

Good luck!
Laurie