First, I am a fan of Newt's knives, so that is my bias. They are very solidly built. They have comfortable handles, and are well designed. They are mostly made of a steel called 1095, which is a good quality cutlery steel and has been in use as such for over 100 years. Newt's kydex sheaths are also first rate. If there is a complaint to be made, they do not come from the shop as sharp as most people would like. Many people,myself included, like knives to be razor sharp from the factory. Newt feels that a coarser edge is sufficient and more durable. The edge that comes from the factory/maker lasts only a very small fraction of the life of a working knife anyway, so I don't really see it as that much of an issue.
The Air Assault is a favorite and is great for someone with normal to large hands. The U.S. Marshall is a slightly smaller knife and is more suited to someone with smallish size hands(this is my opinion only.)Newt's knives are great bang for the buck. They are not fancy, but if you need a solid working knife, you'll love them.