We all know what opinions are akin to, so I'll throw mine in here.
I grew up on a 2280. It was my "first" horn and the one I played on through high school until I got my first compensating horn (a Willson 2950). Even though I've never *owned* a 321, I now prefer the Yamaha 321 over the King 2280 for a few reasons:
1) Ubiquity. There may be no euphonium more common than the Yamaha 321. This is beneficial for a few reasons: Parts/repair are EXTREMELY easy to handle through pretty much any shop/dealer. It also means you're more likely to find a used one at a good price.
2) Consistency. Something Yamaha is really known for is producing a very consistent product from instrument to instrument, which makes buying before trying a lot safer bet. Yes, the King is "American made," but in all honesty the instruments I'm seeing come out of the Conn-Selmer factory are concerningly inconsistent - and that includes high-end pro instruments. I'm talking basic QC issues right out of the box (won't go into heavy detail here though). If you buy a used one that someone can vouch for, it's not as big of an issue, but I'd be careful if you're wanting to purchase new.
3)Ergonomics. The 2280 has all four valves on the same horizontal plane, whereas the 321's 4th valve is a bit more "offset," which I find more comfortable for the pinky. I also prefer the 4th valve slide wrap on the 321. The way the 2280's slide juts out on the side has always made me nervous.
Again, IN MY OPINION, you can't really beat the Yamaha 321 if you're shopping for a great quality non-compensating euphonium. It's pretty commonly the choice of pro doublers as well as jazz players. You can certainly achieve that brighter sound you want with it.
As you can tell from this thread, there are a ton of great options, and I think you'd be satisfied with most of the recommendations people here are giving you!
Sean Kissane
Low Brass Specialist, Paige's Music
Principal Euphonium, Indianapolis Brass Choir
Principal Euphonium, Crossroads Brass Band