I think one influence on horn choice in the US is that most studio teachers forget that what you sound like 5 feet away is very different from what you sound like in a hall. An 8D sounds great and powerful up close but dull and foggy in the hall.
Clarinet players are trained this way too in the US. That's why they all play every piece of music as if it were the 2nd movement of the Mozart clarinet concerto.
Another thing to keep in mind is that an 8D is a low F horn with an added B-flat side. The 8D's B-flat side is only meant for the high register. An Alex 103, however, is a B-flat horn with an added low F side. They can't be played the same way.
Whatever horn you choose somebody will hate your choice. That somebody will have an elaborate and scientific-sounding reason why the horn you chose is a terrible choice -- even if you sound fantastic (or perhaps especially if you sound fantastic). It's all so personal (mix in some caprice and malice).
I agree with Martin that some horns are too expensive for the value they give; but I've played many good Conns, a few very good Holtons, one awesome Yamaha 667D, and some Alexes that were real duds. And I'm speaking as a person who plays two Alexanders (not usually at the same time) because that's the sound I want. One of the best horns I ever played was a Kuhn 293x in yellow brass: too bad I didn't have that much cash on me. How Hoyer sells horns I'll never know -- to me, a Hoyer blows like a bunch of bricks but I'm sure that's my fault.
My philosophy is: if the player sounds great on the horn why fuss over what he/she plays?