While watts may create the same amt of heat the big difference between heaters is distribution, sound, and quality of the thermostat. The quietest heater is likely the oil filled radiator type - love them but they often require a fan to distribute the heat and they are kinda bulky. Most of the inexpensive heaters you find at Walmart etc have fans that are either loud and/or so pathetic that the heat builds up near the heater which triggers the thermostat to shut off while the rig is still freezing cold.
I have used a boatload of heaters over the years. I currently use a Vornado which has sufficient air power to distribute heat quickly to the rest of the rig. It's not as quiet as advertised but it's OK. My runner up is to buy two cheapo Walmart specials and set them to "LOW" and place one in the front/back. If you these inexpensive heaters on low the heat will likely not build up enough near the heater to trigger the thermostat to shut off prematurely.