Confused by greenhouse plastic choices

So, I've got some cattle panel greenhouses with plants that I need to think about covering now that the nights are getting colder.

From what I gather online, I need a roll of 6mil poly.  From there, things get a little hazy.   They say that construction poly is no good and won't hold up and I need stuff that's designed for greenhouses instead.  

Problem is, looking online there are hundreds of choices... plus I get this strong vibe that a lot of the people selling "greenhouse plastic" are selling construction plastic in a new box.  

How do I find some good, reputable plastic that will last?  Are there specific things to look for?
Also, what brands have you had positive/negative experience with?

Have you gone to respectable green house suppliers?

Hi K,
I agree with John, and your best bet for true greenhouse plastic would be to go to a reputable (preferably local) greenhouse supply store/website.

The only difference between "construction" plastic and "greenhouse" plastic (that I know of) is that the greenhouse plastic is made with UV protection so that it does not break down in the sun so fast. Both will need to be replaced eventually, but the greenhouse will give you more years.

Some people prefer longevity, but there are some people who get the cheapest plastic they can and just replace it every couple years.

Basically you need to trust the supplier because you can't tell the difference just looking at the plastics for the most part.

Light transmission level.  Some plastics are down in the 50% or 60% level.  While the ideal is up in the 90% transmission level.  Best quality is the Tefzel type plastics at 95%

UV resistance primarily determines plastic life expectancy.

film thickness and strength also matter