I don't know much about Kayak but it appears they come in open cockpit on closed cockpit. What are the advantages/disadvantages of each?
the sit in yaks ( top picture) give you some more storage options, a little bit of restricted movement, but you're also shielded from the elements a little bit more then a sit on top..
a sit of top gives you a lots of motion for your legs and lower body, the paddle is also more secure with a sit on top. there is easy access storage spots on both types, depeneding on the kind you get of course.. there are lots of options for both types.. I would prefer a sit on top ( what I am currently using). its a bright blue one and I had geese and ducks flying right over me. wish I had my shotgun..
As for stability, sit on tops are usually king is this section. due to them being "able to stand up" they have to be wider. 31" is normal ish, 33" is really wide and so on. The wider you get the slower you will go and vice versa. there are so many brands for the guy who doesn't want to spend an arm and a leg, around $500 is where you will get the best bang for your buck.. to an extent of course. perception kayaks have a 11.5' sit on top, it has a 500# capacity for $640. (what I am using A.T.M.) I can get more brands and what not if I get more time to look while at work today.