Ok, know from the start that I do not deal cases nor affiliated with Marcus Bonna in any way. However, I have been around long enough to have seen my own horns come back to me for cleaning or repair after having lived in many different cases. The MB-3 is absolutely the best choice and I'll explain why.
If you have a horn that you don't care about, or one that is cheap and replaceable, you probably don't need to read any further. However, if you are someone who has decided that a valuable horn is worth your investment, a good case is critical to protecting that investment.
The MB-3 is my preferred case.
The MB-3 is a flight suitable case which separates the bell compartment from the corpus. Perhaps this doesn't seem useful, but if you've ever been on a super tiny plane, you know that no travel case in the world goes in those over head compartments. In this case, the bell can be zipped off and stored over head and the horn corpus can go under the seat in front of you (provided you thought ahead enough to book an aisle seat.)
I can hear the objections now...
"But Jacob, there's not enough space in there for all my stuff!"
You should never store anything inside your case other than your horn. Not pencils, not screwdrivers, not oil bottles, not anything. All Bonna cases contain mouthpiece pouches, which are ok to use, but I can't recommend storing anything else in the side compartment. You do not need your case to fit your mute in, you should pack your mute separately on the rare occasions that you need to use it.
A pouch like this can be had for ten bucks. Mine is a shaving bag that I think I got at Khol's and it contains pretty much everything I would need for an emergency repair as well as all of my maintenance items. I have it clipped to my case next to a Nalgene bottle for water. I can actually carry much more this way than if I tried to pack a bunch of stuff into the tiny pocket inside the case.
What will happen if you ignore this advice?
Luckily, there is an entire industry devoted to repairing the results of a mouthpiece or errant screwdriver playing pinball inside the wrap of your horn. If you keep finding little dings and dents in you horn that you can't identify the cause of, it's your case. Worse yet, the mysterious dings and lines that show up on bell flares can often be traced to the bell flare floating inside the same case as the corpus. Need proof?
This bell is roughly one year old. It has lived in a MB-7 (bell and corpus together) and has suffered for it. I am sure that many MB-7 owners have no trouble keeping their horn dent free, but I find that most professional players would rather have a system that just works, rather than one that requires an extreme amount of care to maintain safely. On a personal note, it saddens me to see horns and bells come back this way considering the monumental effort that I put into developing good finishing and temper control techniques. I want players to be able to enjoy my efforts far into the future and a well designed, cushioned case can really help!
The MB-3 is the best design of case for my sensibilities, especially if you combine all your accessories into a pouch outside the case. It's not terribly expensive, fits many horns (can be on the small side for horns wrapped very big), is very safely cushioned, and is easy to travel with. What more can you ask for?