I suspect you are planning some sort of tunnel. Choice of plastic depends how permanent you want it to be and how big the structure is going to be.
When ever/if I do something that I want to last more than a growing season or a year..I go for proper polytunnel plastic. I've had some from First Tunnels..for my real tunnel and for home made 'things'.
With these sort of things it is case of 'you get what you pay for'...cheap or wrong type of polythene for purpose is useable only for short term.
Current cover on my tunnel has been on for 10 years now.. and other than holes that I have poked through or what is caused by cats claws..the rest of the plastic is still strong and don't show any signs of getting weak/brittle. It has outlived its guarantee by many years...
..and hopefully it will still last few more.