I agree with all the above remarks about the fit and function of the horn. And I will add one more thing to consider.
It has to look good as well. And I am not necessarily talking about a horn that possesses any fancy engrailing, rings and grooves, intricate lathe work on the base plug, etc. I am talking about the overall appearance of the horn, whether it has nice graceful curves, a pleasing taper from back to front, the natural coloration of the horn and so forth. Let's face it, you will only be actually using the horn for maybe 1 or 2 % of the time you own it, the rest of the time it will be hung up on a wall or something. So the horn should make you smile every time you happen to glance at it.