The adjustable shoulder straps help with fine tuning the impact vest to the right size.
Impact Shield size should be the same as your wetsuit. Look at the sizing chart for more details.
The Mystic Impact Shield 2011 is the all around winner in terms of protection, movement and value. It is very comfortable with shoulder reinforcements, flex panels, and it offers tons of rib/kidney and impact support. It also has plenty floatation to assist with self-rescuing and swimming back to shore.
The back has reinforced panels for spine protection. It works with both waist and seat harnesses.
If impact protection with floatation is what you are after, while maintaining flexibility and ease of movement, the Mystic Impact Shield 2011 is a great choice, w/out the high costs.