You want a heater that is 125watts or better, ooooooor two heaters in the 75-80watt range just incase something goes wrong (kinda depends on the outlet availability). I am a firm supporter in eheim jagers or cobalt aquatic heaters.
As far as wattage. It depends on what the average temp in your house runs. In the summer I keep the house at 78 so I dont need a heater and I unplug it. Right now I keep the house at 68. I have a 120 with 40 breeder sump and only run a 150 watt eheim jager.
Big advocate for multiple heaters for redundancy, even with the use of a controller. If one sticks on, the others should stay off. Hopefully gives you time to deal with it.
Eheim Jagers are dang near bullet proof and have a reputation for reliability. It is all I use and they have been a staple in the industry for many decades. Still with the cost comparison of a heater and being potentially deadly, I firmly believe they should be on a controller or at least have some sort of tank temp monitor that can alert you should a $25 piece of equipment fail and could wipe out hundreds to thousands of dollars of life.
I prefer the titanium heaters over the glass heaters for the obvious reason of breakage. If you use a sump with your 40g tank, then I would recommend two heaters for redundancy. But, if not using a sump, two heaters in your DT might not look asthetically appealing...but if you can do it, then better!