I guess the best way to demonstrate the benefit of having a 29 gal. Over a 20 gal. Is a pic of my 29 gal. Reserve tank. I use it as a home for injured or misfit fish. Pictured are: 6" Electric Blue Jack Dempsey "female", OB Peacock, blurry Sunshine Peacock, if you squint. A couple of Convicts. Not pictured, two more Convicts, and a small Pleco.
Three Texas Holey Rocks, Manzanita and Maylasian bogwood.
Now that's a full load. It's been up and running for five years. I couldn't get half of that in a 20 gal. There is not that much of a price difference between a 20 and a 29. Regarding aquariums,
Its better to go bigger. Note: the second pic is the same Tank with four small Texas Holey rocks. Had to take one out to accommodate a pair of wayward fish.