I think either would work personally and run about the same risk of failure/malfunction. I can think of lot of bad reasons to use a submersible heater just as well as a pad heater. The thing is getting a set up that keeps the water temp preferably 78-82*and stable. If heater pads work for you then buy them and if a submersible works for you then buy them. There is more than one way to skin a cat as the saying goes. Get a thermometer no matter which one you go with though. That way if something goes awry you can be alerted before your fish let you know that they are cold.