I would opt for the Bad Boy if I was comparing the two myself. I know some Bad Boy owners that love them, but be warned, they are hard on your back. Haven't met many zero turns that aren't though, without the suspension seat upgrade and or shocks like on the Ferris as mentioned previously.
If you do get the Bad Boy, I'd probably go with the Kaw engine. Personal preference, but I seem to hear more positive than negative about them vs Kohler.
Have you considered Scag? Biased of course, but the Patriot comes in right at 6k cash offer, and is a great machine. 3400 hydros (upgrade over the maverick), disc brakes, and the Kaw FX series engines. Guaranteed to last the homeowner most of their life, lol.
If you don't want to spend as much, there are still plenty of options that are a little cheaper, that will still be good machines. Depends on your usage requirements and the amount your mowing I guess.