not always... 250 watt does my 55 gal really nicely... for the 100 gal you might want to get 2 150 stealths and put em on opposite sides of the tank for better heat distribution
Go with an Ebo-Jager. I've had mine for 3yrs, and it has sustained all the abuse that my midas has dished out. The cheap ones tend to crack if u have any aggressive fish.