IIRC the Zytel handled ones have blades made from steel with lower edge retention (AUS4?). There have also been some reports of quality issues with the Zytel handled models. Again drawing on memory, I believe that the more expensive aluminum handled models have AUS8 blades and have not had the quality issues.
My advice would be that if you are going to buy a Zytel model, that you try to handle the knife you are going to buy prior to buying it so as to ensure you get one with positive lockup. Another option would be to buy from an online dealer who will check for you. Some of them will. Some won't. It would be worth paying a bit more to contact the dealer prior to the sale and ask him to check for you.