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It is advisable to file your I-140 with indication for a CP because it allows you to file for both options (AOS or CP) later on without any additional delays.
If nothing is specified in the filed I-140, the AOS option will be approved by default on your I-140 application. If you decide that you would like to go for CP after your I-140 is approved for AOS, you will have to file another form I-824, which takes 3-6 months to approve. However, if your I-140 has a CP option and you decide to go for AOS, you do not need any other form to file, you just simply file an AOS application. I think that your lawyer is maybe hoping for some extra work and money for him.
You still have a chance to ask for the CP option on your I-140 if your lawyer sends a letter indicating this right away.
Good luck!