I can see where you like the way Bad Boys are built. I too have a love affair with them. I looked at the ZT with the Briggs ELS and the Kohler Courage. Then I saw the Pup and Lighting. But I did a lot of research on the ZT. I never saw a complaint about the ZT. Still haven't! I visit the Garden Web Tractor forum a bunch. Have for the last 12 years. If there's a problem with a home owner mower or engine, it's there.
There's page after page about failures on both. But then there's success about both also. I think it depends on maintenance. Change the oil, the oil filter, and the air filter, the life of the engine will be longer.
One thing I do know after going to the Bad Boy web site is this.
26 HP BRIGGS & STRATTON ELS
724 cc, 33 ft. lbs. @ 2600 RPM
27 HP KOHLER
725 cc, 45.5 ft. lbs. @ 2400 RPM
That's 12.5 ft. lbs. more Torque for the KOHLER.
To me that's a big difference. Heck! My 30 hp Kohler Command Pro only has 47 ft. lbs. of Torque with the same displacement.
Think about it a while. I feel you'll make the right decision.
Oh! I got the Pup with a 30 hp Kohler Command Pro. Yes! I'm very happy!