Last summer I had about 12-13 residential lawns. Only one of them had a gate to the back yard. It is about 30ftx40ft. I used my trimmer on it everytime. I tried using a 21" but it actually took me twice as long when I used the 21" rather then the trimmer. What purpose would the push mower serve???? I used my trimmer on a back yard and any hard to reach places just fine. Even with commercial accounts that have small islands. Seems a steady trimmer hand can get the job done just fine. Do yall use your 21" alot?