Good choice in fish ;D ;D ;D , anyway , do you have the discus now in the ten gallon??? If u do i would refrain from moving it for a while bc discus are very sensitive towards changing of the surroundings but, if you do not have the fish now and u are getting it from your lfs to put in one of the tanks i would definitely put it in the 20 gal. Also how big is the fish if it is larger thean about 4 inches or five it hould go into the 20 bc the ten would be too small.*thumbsupsmiley*
-Daniel
Benestol