BUY THIS ONE!!
I don't normally write reviews but I wish someone had left a review to forewarn me of some things to save me the expensive lesson I learnt.
We have two hyper and slightly deranged dogs who had trained us brilliantly to get up and open the door every two minutes. After learning the new meaning of a "nice walk in the woods" I realised I need the rest at home even if they don't so I looked into dog doors.
Due to price (in comparison to the aluminium version) I bought the Staywell 760 in a large. It is solely made of plastic and once the dogs learned how to use it they were in and out like rockets. They chased each other in and out without slowing down even slightly for the flap. The dogs conducted numerous experiments on whether they could both get through at the same time and needless to say after about a month the flap broke so I got a replacement flap (£14). To replace the flap you need to remove the whole unit and is a pain in the backside. After three months the actual unit itself had split and so I knew it had to be replaced before it fell out. Also I should mention the flap is VERY noisy, as in loud enough to wake up my mum and dad when they slept over in the back bedroom (above the doggy door). It clatters at least three times as it swings back and forward before closing after a dog goes through it. Sooo annoying.
So, I shopped around and decided to go for the "heavy duty" Aluminium Staywell 640. I cannot tell you the difference in what you get for the extra money. The whole frame is metal and a lot neater and less bulky than the 760. It fixes with four very simple screws as opposed to the 8 fiddly screws of the 760. The flap is flexible but heavy duty rubbery plastic, it has draft excluders around the edge of it and a small magnet along the bottom that brings it to rest at a closed state. I can see that should I need to replace the flap then it just unscrews without removing anything else (although the only way I can imagine this flap getting damaged is if a dog chews it) and best of all - it's whisper quiet.
The only plastic bit is the door that slides in to stop the dogs going in/out. But even this is not flimsy like the 700, it's very solid and has a locking pin so nobody from the outside could remove it and get in the house.
I would give the 700 series 1/10 and after a month with the 640 still like new I would give it 10/10.