why is it better?

many thanks for the response.
however, i am still a little bit confused.
so you are saying i had better use a entry in the new visa stamp, right? and i cannot do the auto-revalidation, right?
actually, i am leaving for china several days after i come back to US from canada. i will return to US for the new semester in august. the I-94 in august is important, but i do not know if it is important to get a new I-94 when i am now back to US from canada.
looking forward to hearing from you! thanks!