I opted for the jbj arctica 1/10th for my 30long... I went soo large because the ambient temp in my apartment will reach about 100 degrees in the summer, and i won't be there to help a smaller chiller keep the tank cool.. (my tank is at college and i am home for the summer). So far it hasn't let my tank rise .1 degree above the chillers "on" point. (ac3 controlled)
If i were you, get what you can for the 1/15th, and buy a 1/10th.