In a world of ideals, or if at all possible, carry a fixed blade. It follows the KISS principle, is inherently stronger, has fewer parts to break, and is hands down preferred over a folder. I make it a point to keep a fixed blade (Mad Dog ATAK2) close by in most of my travels.
If you cannot have a fixed blade, you enter the world of "tactical folders", a term I am tempted to describe as an oxymoron. However, there are many good folders out there. Just realize that you are generally sacrificing strength, simplicity, speed in deployment, and ergonomics for concealability and convenience.
Spyderco was the first to offer a quality folder that was fast to deploy and had a reliably strong locking mechanism. However, Benchmade has definitely upped the ante with the excellent Axis lock, and some very nice automatics. Chris Reeve's Sebenza is also a solid, well made folder with probably as bulletproof a lock as the BM Axis.
Ultimately, try out the major brands. More importantly, get good training; test the knife that you will be carrying for ergonomics, lock strength, etc., Remember that you will be using your knife more often for utility than defense so keep that factor in mind also. And don't forget to check the knife carry laws for your locale.
Edged Weapons Conference 2000