go with the 18 for now untill you build up your supplies and ambition to keep the larger fish.... almost all ca/sa cichs do great in 18's 24's that much more water to keep bigger is sometimes pointless....id wait untill the right situation comes for the 24 such has the time for your 18
My preference is for the 24". Why? Because you're less limited in your decision of what to keep. At least you have more availability and flexibility in tank set up with a larger option. Good luck in your decision making and keep us posted.