600nm is a piece of cake in a Mooney and you can do it nonstop even with standard tanks. I know some might think I'm biased, but honestly I find the Mooney more comfortable than my friend's Bonanza. The Bonanza looks more comfortable but really isn't. Yes, you sit higher and leg goes vertical ahead of you, however, you can't stretch your legs forward because the rudder pedals are practically under you as well. It's nice that there is no center console so stretching sideways is possible but you can't put your legs forward. It makes me feel like flying economy class with the my bag under the seat so I can't stretch my legs. 
 
In my M20J, I can stretch my legs forward or bend my knee, or slide back and put my leg sideways toward the middle. I think the variety of positions that you can sit in is key to long term comfort. I've flown 5 hours, 6 hours, and 8 hours nonstop. It's doable. 
 
The panel (M20J and forward or retrofitted earlier models) and systems are simpler. The weight/balance is simple, the fuel tanks are simple. The Mooney is simpler and more logical. The Bonanza (at least the model I've flown in) adds a lot of complexity (such as having 4 separate tanks or a changing CG) but no benefit over the Mooney for it. Worst yet, the useful load on that Bonanza is worse than on my Mooney. So not only does the Bonanza burn more fuel, take less load, not go as far, but the whole "more comfortable and more cabin space argument" really doesn't float. I haven't sat back seat in the Bonanza but visually it looks like less leg room for backseaters than M20J as well. 
 
For your specific mission a Bonanza would be a viable substitute but that's my four half pennies about Bonanza vs Mooney.