I've had soft covers for years and never had an issue with theft. In reality if they want in they'll get in and smashing fiberglass with a hammer is not much harder than cutting a soft cover.For a hard cover, I suggest Undercover. They have three models: Classic, SE, and SE-Smooth (must be painted). Great bang for your buck.
I'm not a fan of Bak products. They are not known for quality.
The Access covers are very good, but there's no security since they are made of vinyl. The Access Lorado is the most popular model.