Just as important is the type of heater, the proximity of it also matters. On some heaters, such as the rods that get too hot, many people want them to be further away sometimes to they don't get too hot and you have to move around the saunas like a rotisserie chicken. Some heaters are exposed where you can see the carbon panels and others are behind wood. Some heaters have a space in between the wood you rest your back up to and the heater behind it so sweat doesn't get on the carbon heater. Salt residues that drip in between the wood edge and then go onto the panels is not what I prefer. I like that some saunas have this gap in between.
Most infrared saunas have the carbon heaters exposed and are in front of the wood. Some saunas have their heaters in the front where some only have the heaters on the sides and back and some built into the floor to get the feet. Many people appreciate this on a cold day.
The biggest thing that separates the saunas is how high the heaters go up. I do not promote the saunas that have heaters go up to the roof of the sauna. This makes the sauna hotter but doesn't deliver the far infrared as its too far away above my body. The height I look for is around 26 to 31 inches. If someone is taller, the higher the better but if too short, the heater can be below your shoulders not giving a full surround feel.
If the sauna is not a low EMF saunas, I strongly suggest an EMF blocker you can wear or a machine thats transmutes it.