That would depend on the size of the area you are going to use it in. If you need it for 2 trays, the 20x20 would probably be better. It would only require one plug outlet.I know cost wise the 10x20 is cheaper.

Personally i use the 10x20 size, since it dont require as much space. But i use twice as many of my electrical outlets.you have more options as far as space available to put them.

the brand i use it hydrofarm, stay away from the harbor freight ones, they are not rubber coated.they are more like a water resistant heating pad.

i would probably reccommend the 10x20 size mainly because of space. if i did'nt need 2 trays at once, you could also spread the trays out better if you had 2 mats and they would'nt be scrunched up together.

DW ordered a hydrofarm CK64050 for me a few days ago. I also threw in a bulb planter/weeder to get the price up enough for free shipping. I know nothing about any of the stuff ordered. It is to be here 2-14 to 2-20. I have a small one I made a few weeks ago, it draws 19 watts, the one ordered draws 17 watts.

Larry