I buy the cobra expedition because he was the fastest plastic (read "good for abuse") kayak I could find. He keep up with most "sit inside" sea kayaks and beat the hell of all the other plastic sit on the top ones. Other point of interest are those huge hatches that can swallow a 25kg kingfish to get them out the way of our toothy friends.
A low drag hull is a safety and endurance factor: If your kayak is a slow and stable design, once loaded you will burn yourself to get to the diving spot, and risk to drift hopeless if the wind increases. The expedition is the only plastic SOT I've try that require a minimum amount of energy to keep moving and that enable me to face changing weather conditions.
Is true that because is narrow, is not as stable as other designs, better try before buy and need a adaptation period.
The Scupper Pro is a well proven value, but he doesn't have the same range and will struggle with a bit of wind.
Sometimes the expedition price seem up, but keep in mind that he comes equipped with 3 hatches and a ruder system...
The point is to know the real use you go to give to the boat: If you want something to reach a close by spot on generally good weather conditions, something like the scupper or the cobra tourer is enough, but if you want to increase your range on good safety conditions you better look at a fast kayak.
Don't forget the paddle... they have the same importance as your fins when diving: they transmit your energy!
The fish and dive is a nightmare to get moving: so wide that there is no way to paddle properly and so much drag that you can only use it if there is no wind, and have plenty of time! On the good side you can stand up, mount a small engine and even take a small passenger. The hull was design a a tandem to play around the beach, and I use to have 4 of those for my clients... to avoid unless you have really good reasons to get one.
There are good fiberglass or kevlar alternatives to the expedition, but too expensive for me! and in NZ we don't see all the "plastic" brands to find another choice.
listed in here you'll find reviews of most of the kayaks:
http://www.paddling.net/Reviews/Kayaks.phtml