Best tank for a reef is the largest one you can reasonably afford to set up & run..... The more water volume you have, the longer it takes for minor problems to become major ones, and gives you more time to catch them.
As to your other question, personally longer tank is better than taller.... taller tanks require more powerful (read: expensive) lights to "punch" down through for proper coral growth.
Wanna calculate the cost of that new reef tank?? Take your worst nightmare, double it, and you're halfway there.