At this time of year with the temperature fairly moderate, it is a good time to check what you may need in the way of heaters. I feel the ratings don't do a very good job of figuring in all the other heat sources we may have. Filters, powerheads and lights all add heat. Depending on how much you have to keep heat from moving out of the tank, you may not need much heat. I currently have no heaters connected on larger tanks as they begin to get too warm from just all the other heat items. I would suggest just trying the smaller heater and watch if it keeps the tank warm enough. A 75-80 gallon may take 12 hours or more to cool enough to notice so it will take some time to figure if you need heat at all at this time of year. A smaller heater is safer if it ever sticks on as it will give you more time to notice before killing the fish.
I run a 200 watt on my 125 and find it is just enough when the room temperature drops to around 68 during colder times. Now when I AC down to 74-75, I find the tank temperature creeps up above 76 without heaters plugged in.