20" is usually fine for electronics components. It is not usually fine for computer components. If you are planning on putting in a rackmount server (HTPC) or storage array, best to know what cases you'll be using before deciding on 20" or 26". Also, the 20" MA rack I have(AX-SXR) is more like 19.5" useable. I had to slightly modify 20" rails for my disk array. I do have about a 5" space available behind my rack. I'm glad I do too for ventilation. Those components get quite hot. I have a cooler guys Fan Controller at the top of my rack (http://www.coolerguys.com/840556091677.html
) which is hooked to a 12V relay that runs a Fantech inline duct fan (located in unfinished storage area) that exhausts all the heat. Works great but only wished the coolerguys 1U faceplate came in the same brushed aluminum as the MA faceplates.
If you are interested a HTPC or storage array, here are the ones I use in a 20" rack.
Silverstone Black Aluminum HTPC case. Loos great because it matches the brushed black aluminum of the MA faceplates.http://www.newegg.com/Product/Produc...82E16811163117
Norco 12 Bay hot swappable Disk Array. Shallow depth, holds 12 drives (36TB) and is only 2U.http://www.norcotek.com/item_detail....0db23094bf2ba7