A lot depends on whether you're going to be using this knife for anything other than defense.
Me, I don't like having anything banging up against my chest. That includes flashlights, too. For defense, you don't really need to spend a lot of money. I've seen a number of very nice neck knives on eBay that are skeletonized with Kydex sheaths. Buck and Cold Steel have some very reasonably priced neck knives. If you're not going to be using the knife for other purposes, you don't really need to invest in a super steel model. You can get 420HC and AUS8 blades extremely sharp. The big question is, how much are you willing to tolerate in size, weight, etc.?
You can even get a Kydex sheath for a Spyderco Endura. Just hang it around your neck and get one of the new Spyderco light weight Enduras with a flat grind blade. That way you'll not only have a quality neck knife, but one you can use for both self defense and other tasks.
A very nice small neck knife is the Buck Smidgen. It's tiny, but will come through in an emergency.
Years ago I watched a great scene in the movie, Cliffhanger with Sylvester Stallone. The bad guy was trying to step on the good guy's fingers and send him plunging down hundreds of feet to a very nasty death. Ah, but little did the bad guy know that the good guy had a Spyderco knife clipped to him, and he drew it, opened it with one hand and plunged it into the bad guy's femoral artery. Next scene is the bad guy falling off the cliff.
So are you going to be using the neck knife as a last ditch weapon? Obviously if this is the case, you'll most likely want something you can pull like a rip cord and use immediately. If that's what your purpose is, you'll want the largest and sharpest knife you can wear comfortably. I'd also give serious consideration of the Cold Steel Secret Edge neck knife. At 2.4 ounces it's very light weight; it also sports a 3-inch blade that's very capable.
Again, you don't need to spend a lot of money on a neck knife. Edge retention is not a consideration if it's a last ditch weapon. The knife only needs to be sharp once, and that's when you need it. If you're going to be using it for other things, then naturally, edge retention will need to be a consideration. But AUS8 should be fine for most situations. It can be sharpened to ungodly sharpness and if you ever do need to resharpen it, it's easy to get the sharpness back.