The 720 is a good knife, but it does not fit my hand well, the only Mel Pardue design I'd say that against. But if your dealer has it, go handle it - if it fits your hand, it's a hell of a knife. My fiancee loves hers. :thumbup:
OK, helpful mode.
Of the models you listed, I'd point you at the Resistor, although there's nothing wrong with either HK.
The Griptilian is great knife, a world beater in bang for the buck.
But.
The reason I no longer own a Griptilian is that I own a 730 and a 710 and a 520 and a 720 and a 921 and a 940 and a 770 and a 705 and - well, you get the picture.
The full-built Axis locks are NOT better values than the Grips, but they ARE better knives - at a higher price.
You sound like a guy who is already well infected with knife-itis.
So I'm going to guess that once you're bitten with Axis-itis, you'll eventually want more than one. So if you're willing and able to spend the extra forty bucks or so, I'd go ahead and start with a full built Axis, rather than Zytel.
Based on what you say you're looking for, I think of the knives Benchmade still makes, I'd encourage you to look at the 520, or the 710 - very hard working knives, with excellent ergonomics, and great cutting blades. Both are very versatile, tough, hard workers, with good potential for last ditch self defense.
If you're sure you want combo edge, I'd get the 710. Otherwise, the 710 is pointy, easy in the pocket, comfortable in the hand, and has G-10 scales. The 520 has a tough drop point, a high slicing grind, a VERY secure grip, that fills your hand (but also, I must warn, most or your pocket) and has aluminum scales - more rigid, but more prone to feel hot and cold with the weather.
If you want a knife ALMOST as tough and useful as those two, but much lighter and slimmer, look at the 940 and 943.