Hello Vince,
Normally I'd steer you away from the RZT series due to the issues I've read of over the years with respect to the ligher-duty Hydro-Gear brand EZT drives they use, but it looks like for 2011, Cub is putting the ZT-2800 series drives on the RZT 54". The 2800's are fully serviceable with spin-on filters. Kawi's are top of the line engines as well. I've read that they're not rebuildable anymore due to crank bearing design, but I'm not sure if that's true or not. There's not much that you can rebuild anymore anyway.
I'm very happy with my 2010 model year Z-Force 44". They discontinued the fab'd 44" this year and went with the 48" for the smallest. If I were you, I'd seriously consider the Z-Force 48" for your needs. You definitely don't need 54" for 2 acres.
Joel