You may however want to go with 2 - 150's. Having 2 is always better just in case one of them goes or gets broken. Especially with aggressive fish that might decide to take out their feels on the heater.
If your place doesn't get that cold, two 100's will be ok.
You need 3 watts per gallon, or for the 75g, or 225 watts total.
I agree that two heaters is best. that way if one heater sticks to the "on" position, your fish will survive.
I'd get Ebo Jagers over Rene Cal Excels any day. Ebos are actually cheaper, too. (From That Pet Place: 100w Ebo @ 16.49 vs. 17.90 for a Rena; 150w Ebo @ 17.49 vs. 20.60 for Rena). Pretty easy choice, for me......
I've got 2-200 watt heaters on opposite ends of the tank, one near the intake and the other just under the spraybar...both are visi-therm, I've never had a problem with either one of them.