Hi ethies, fellow Iowan here.
Our state law can be a real pain in the butt when it comes to what you can carry/not carry. For example, I'd love to own any number of Busse models, however most of the models I like start at +7" blades.
For a SD weapon, a knife is basically a last resort weapon. In your example of being chased, a cell phone call to 911 would have been the better solution. A knife is not an answer to someone armed with a gun. Also, if you stab someone, even in self-defense, be prepared to face prosecution.
If you MUST carry for SD, your best bet is to stay within the jurisdiction of the state and local law (>5", no daggers). That being said, a folder is significantly easier to conceal; and posting your question about what knife to buy in General Knife Discussion>Blade Discussion Forum will yield you a better response as to what other knife users recommend you carry. Remember - most crimes involving knives involve simple kitchen cutlery; so when it comes down to a real SD situation any bladed weapon will do.
I personally think the Columbia River Knife and Tool (CRKT) M21 makes a fine SD carry knife. The M16 model they make is a bit more tactical (they even make "military" and "law enforcement" model M16s) but would also work. If you live near Des Moines, or Iowa City; check out Scheels selection of CRKTs.