Best Options & Considerations for a 120v Electric Heater?

[Rivka]

Although i will give my specs in a sec, i think this is a useful general discussion for any folks looking to heat and that don't have access to Natural Gas or a 220/240v power source.

So personally I'm heating a 4x10 lean to greenhouse that has the 10ft side "leaned" over a wall with the sliding glass door to a heated room on my house and actually has another 4ft wall against another side of the house, the house is brick. Mine has a surface area of 168sqft and a volume of around 240 cubic feet with 4mm twin wall poly. I am also looking into secondary wall insulation. I'm in a 7a zone, so it can get down into the teens, I have intermediate orchids for the most part so will aim to keep thing over 60 at night if I can. I do plan for a back up propane heater for power outages that are an annual thing around here and my collection is small enough that i can drag everything inside if we have a truly cataclysmic winter. I will have a humidifier, it gets wicked dry here in winter.

My math on the calculation sites has me needing between 8k and 11k BTU depending on who I ask and one place translates that to 2 Kw electric

sites i found useful to figure those sizes and amounts out were:
Greenhouse Surface Area Calculators
and
Greenhouse Heater BTU Calculator
They had the best info to figure out all the details, but their overall numbers were lower than other simpler sites I calculated at.

General questions:

1. How do the various types of heaters (IR, Ceramic, Forced Air) compare when it comes to efficient heating, affects on energy bills in models with similar heating capacity?
   
2. I know in many cases you get what you pay for, but I also know that things marketed to already costly hobbies tend to have a "passion tax" markup on them. What are folks' feeling about some of the more costly heaters over the 70$ models that are everywhere? What are you getting for this sometime huge price difference? This is assuming that we are all talking about heaters that are sold as outside and/or greenhouse heaters in some way, not cheep-o desk heaters. a good example of these two types are here: Charley's Best 120-volt Heater vs 120v Portable Heater both these are very similar on Amps, Watts and translated BTU's
   
3. Heat that is possibly more or less healthy for plants and/ or people? do any of these heater put out gasses as a result of their tech that bothers orchids?
   
4. Is there a standard for translating BTU requirements for a given space to Watts? or does this vary with the type of heater tech used?

   Here you go: 1 watt equals 3.4 BTU/hr Watts to BTU conversion calculator Thanks David!

5. Brands folks have consistently liked or had problems with?
   
6. other consideration that i need to think about with heaters that i haven't mentioned?