What is the best mobile terminal?

That depends…what is the application(s) running on the device, the environment, what is being scanned, what type workforce will be using the device.  The application determines what is the best terminal to use.

How do I determine if pistol grip or hand-held form factor is better?

Deciding how the terminals will be used (how much scanning vs. how much keypad / screen entry) is key.  More % scanning pistol grip is probably a good choice.  Some manufacturers make devices that allow you to attach / remove the pistol grip as you choose.

Why are the industrial devices so expensive compared to smartphones and consumer tablets?

Rugged/enterprise designed terminals are built for 7 - 10 year life.  They also include features that often make works more productive compared with using a smart phone or tablet.  This will often lower the total costs of the devices below that of consumer devices.

Why is the OS / technology of enterprise devices behind that of consumer devices?

While this is beginning to change, most manufacturers have stayed with "tried and true" OS platforms because many of the applications being run on these rugged warehouse devices are still menu-based/transaction-based and do not require (or take advantage of) an enhanced user interface.

Is purchasing a "wearable" terminal a good idea?

Wearable data collection terminals enable the user to be "hands-free" for material move and picking operations.  There are a couple of new solutions available that might make this viable for you, including new Android OS devices with larger, higher quality displays.