To pick a heater for your aquarium, very simple. Most heaters nowadays are fully submersible. I think it's rare to find a hang-on-the-back heater. Those were around in the 80s and 90s, but now everyone uses submersible heaters. The only advice I would really give you is that you put the heater low in the aquarium. You don't want to put it in an area where it's going to be exposed to air when you do a water change. You don't want to have to remember to unplug the heater every time you do the water change, because the one time you forget, the heater will crack if it's on while being exposed half to the air and half to the water. That difference in temperature, the glass shell on the heater cannot take and it will shatter and result in a big explosion. So, always put your heater low in the aquarium.
The only other heater that you can really consider are under-gravel heaters and those are used in planted aquariums where you want to simulate root growth by promoting convection currents by heating the area underneath the gravel. But, for most people's aquarium, that's overkill and it's expensive. It is if you have the money and you don't want to look at a heater in the aquarium then you can look at an under-gravel heater, but most people are fine just putting one heater in the aquarium. You stick it in the corner. The only other rules I would follow is to not put your thermometer right next to the heater, because you're going to get a false temperature reading every time the heater is on. So, put your thermometer on the opposite side of the aquarium that you put your heater. With any other piece of equipment, you always want to unplug it before you service it, especially a heater. If you take a heater out of the aquarium while it's been plugged in, or if you unplug it and pull it right out, it's going to sizzle like crazy as the hot glass is exposed to the air. Always unplug it and leave it unplugged for 15 or 20 minutes before removing it from the aquarium.