My Ferris has full suspension and I can cut 'fast'. I used an exmark zt with just a suspension seat and speed was limited by the bumps. If I carry speed I can mow the steepest parts of my lawn even with a little 'dew' on it. If I stop mid slope and the grass is damp at all, it will spin the tires. I don't stop mid slope and I don't mow when it's really wet. If I used my B for mowing, I still wouldn't mow when it's wet.
I went back to read the original post, he talks strictly about mowing, no mention of needing the BX for anything other than mowing. In that scenario a zt wins outright every time. Of course, like I mentioned, the BX can do so much more than mow, and mows nicely, just slow.
I agree with GreensvilleJ, if you need both, both is the way to go, especially for 15 acres. I agree with it so much that it is what I personally do. Ferris for mowing, big B for everything else. I did not purchase a mmm for that reason.
Henro, I'm jealous (yeah, I know it's not a good quality). One of these days I'll get to retire, but for now saving about 80 hours/year in mowing time is priceless. I only mow about 2.5 acres. I couldn't image the time savings for 15 acres.