I use the Tropicool poly, its somewhere between 95-110 ltrs and has held block ice for more than 4 days, plastic bottles frozen last longer. I use it in my boat as the seat at the back of the boat, it also has the measuring sticker on it for sizing fish and has two bums at 100 kgs each sitting on it all day. It gets kicked dragged about and some times has bait/fish cut up on it. It gets a hole load of rough treatment that a fibreglass one just would not cope with.
At the end of the day would 4-5 days of ice cold be enough, and how much abuse would the thing get, and would it be used for a seat in or out of the boat. Would it be transported sucurely or just thrown into the ute or boat. If it is going to get any or all of the above treatment the poly is the way to go.
I have had my poly esky for 5 years now and never had to repair it, and it cops a fare amount of abuse. Also be aware that some cheapo poly boxes have very dodgey hinges and lid hold down clips.