what is difference between E-file I-140 and Paper file I-140, Th

What is difference between E-file I-140 and Paper file I-140, Why my attorney only offer paper file I-140, and tell me the E-file I -140 have to wait until CIS get mailed package. It is Perm basic. Dose that mean it will take longer time than E-file because CIS need input my application form into their system, and E-file doesn't. Am I right?. Thanks.
Thanks Pjing, I have read the information in your BLOG, but I didn't get answer for this question.