There is a question on the I-140, where you need to select how do you want to proceed: CP or I-485.
If you decide to do I-485, your file stays in USCIS. Changing your choice to CP after that is difficult and you will need to file I-824 - Application for Action on an Approved Application or Petition. Just looking into I-824 and moving the file to the State Department will take a few months and this is going to nullify the I-485.
However if you have decided upfront to go for CP, you should be able to file I-485 later and both will go in parallel.
ned:
All my friends who did CP have long forgotten the pain and are going soon to be eligible to apply for citizenship. If you can get out of TSC, do it.