I use my 21s on my smaller lawns (less than a quarter of an acre) and also ditches and through fence gates where no mower can fit. i even have a half acre back yard I have to mow with my 21 because the gate is not even 26 inches wide. I use to have a John Deere 48 inch rider that I found too big for most of my yards, plus it kept having issues (bought it used) so I sold it. Now I only use my 21s because most of the yards I do are so small
I am getting a 48 inch Exmark Turf Tracer or Toro T2 hydro next year to do most of my larger yards and eventually a Toro grandstand. Walk behinds are great for hills imho. If you get a 36 inch be sure to get a floating deck. It makes height adjustment much easier.