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.