Here's my opinion, and bear in mind it's worth exactly what you paid for it. Ultimately, you have to make up your mind and live with that choice. But, if it were me, here's what I'd consider;
You'll need something that could be used with either hand, and conversely, something that you can open with one hand [in case the other hand is injured or otherwise "occupied"]. Something that is cave man simple to operate in case your mind is slightly paniced [pretty normal when you think you might die]. It will also need a fairly grippy texture - your hands may be wet, sweaty, coverd in yours/someone else's blood, numb from cold/snow, etc. etc. etc. - so good gripping surfaces are a consideration.
If size and concealability is the consideration, then a folder would be required. In the summer months a fixed knife can be hard to hide. Conversely, a folder will never be as strong as a fixed knife. Many I know carry a fixed in the late fall/winter/early spring, but as the weather gets warmer switch to a folder out of nessecity.
In the folder category, I would/could recomend the Cold Steel AK-47. Wave type knife, and with perhaps 20 min of practice you'll have mastered being able to wave it out of your pocket and immediately be ready to use it for either a thrust push or with a two finger change of position, standard use.
If you can put in the time, then the Spyderco P'Kal is also fantastic. The downside is that the draw is more complicated [involved rolling the knife into your grip] and it is made for a specific style of use. The upside is that it's use is even more violent and instictive, so once it;s out adn ready it's truly cave man simple. You'd have to practice this draw for a couple of weeks to really have it down to a "do it no matter my frame of mind" use though.
In the fixed blade category you have a myrid of choices here, but I'd say you'll get exactly whatyou want by getting a small fixed blade off the boards here from one of our resident custom knife makers - many of these guys are experts at their craft and can build you EXACTLY waht you want not in in size, grip, and use, but also something that si equelly pleasing to the eye. Many of these guys are really great on their prices too. I'm currently having a new one built by Pohan Leu of Leu Custom, and the man's integrity is second to none. At every step of the way he has exceeded my expectations and over delivered. I know many of teh other guys on the forum here are also very customer orintated.
Hope that helps!~