"I'm looking at moving up to a 100/120 Gal tank"
Take time to review your waters current, inline heaters are great, you can actually use less wattage to heater then submersibles. However if your tank is deep, you will need to add additional current to help mix the water for even temps.
Water coming out of your return tube will stay on top of the cooler water. On my 150 gallon I added a power head and a DIY extension for the intake tube to reach the bottom cooler water of the deep tank. I used a few thermometers (cheap investment) placed at different levels and sides of the tank to review the progress until I finally got the whole tank to flow properly at the desired temp.