If you are going to go for protection wear, and the metal security it offeres, why go cheap? The difference between the pants is as follows:
The underpant will keep your butt warm, and save your hips from a 3 foot monster kicker fall. It will be less noticeable while riding to you. It has better movement.
The Armor pant will keep your butt warm, and save your hips from a 5 foot monster kicker fall. It will be more noticeable while riding. But it will dissepate the impact force better thatn the first model.
The question I have is: How do you take a leak when wearing them? :P