I looked it up and the only difference I found is the engine. The MZ61 is powered by a 27 hp Briggs & Stratton engine and available in the USA. MZT61 on the other hand is sold in New Zealand and Australia, and has a 24 hp Kawasaki engine. The latter is more like an entry level zero turn mower. With it's much more powerful engine, the MZ61 suits heavy duty work better.
Both mowers come with 4 year warranty