One big thing to consider - what do the rest of your section use? Different makes of cornet have different tuning issues, these aren't so noticeable if all the cornet section are using the same, but if they're using different makes there will be real problems. Most top bands buy 10 the same for this reason. (There are also issues with certain cornets and certain makes of mouthpiece, but that's a whole new topic in itself!).
Leaving that aside, large bore of any make is definitely the one to get, the difference in sound, especially on higher registers, is noticeable. (I find thin bore ones get screechy on top A and above)
You may also want to consider buying a good secondhand one and having it reconditioned if necessary, our band has done this and (as long as it's a quality job) the results really are good as new, and for less money.
Personally I've used a Sovereign of some sort nearly all my playing life, and although I liked the Cortouis my previous band used I think they're only for bands who have all cornets the same, if you mix them and Sovereigns they don't get on tuning-wise. I've never played a Yamaha for any length of time, but the Maestro seemed well made.
For the last 8 years I've used a Sovereign 928GS that's about 12-13 years old, and it's the best cornet I've ever had, the valves in particular are excellent, and it just feels heavy and well made, all it's ever had replaced is new valve felts (too many semis
) and a trigger spring or two.
Most music shops will let you trial an instrument for a bit, after all, they want you to buy it!
