personaly I perfer a soft tonneau only because you don't have to have much forthought if you need to haul something. You can just roll the cover back as apposed to needing to remove and store it in the case of a hard cover. A lot of soft tonneaus have a problem with the stiching comming apart where it connects to the railing. especially the one sold by toyota at the dealership. THere is a company called 'truxedo' that makes a decent cover. A buddy of mine here at work has had one for a few years and it's held up great.
Theres also the hard roll up kind that rolls up into a can similar to an overhead roll up door. I like those but I don't like the 6" or so the can takes up of the bed. But if you're looking for somthing more secure than a soft cover and not as inconvenient as a hard cover, that would be the way to go.