i remember answering this same question the other day.
from my experience with the 400ex shock is that its relatively easy to install raises the back about an inch, and its super comfy.
the yfz shock requires more fab work, and it raises the back alot.
i will let someone who has a 450 shock give you more details, i have never owned one just going by what i hear.
but i went with the 400 ex shock because i didnt want the back sitting so high and the feeling of falling forward.