I have a ZD21, myself. It has over 1000 hours on it and all i've had to go wrong with it is the hydrostat line had a leak in it and then warranty took care of it. Then 2 more times the other line got a crack in it. This is the only thing that i've had to go wrong with it. Another good reason to get the diesel is you can get the non road taxed diesel. When regular diesel prices are 1.50/gal, you'll pay .50/gal less. basically $1/gal. PLUS, diesel is more efficient than gas is, so less consumption of fuel.
I odnt think you could go wrong with the ZD28, but i dont think you need to spend the extra money on the 7 extra ponies, unless you're going to mow alot of accounts that are 2 weekers. Other than that, i'd go with the 21ZD. JMO