Company A [Consultancy] PERM Approved, I-140 (regular processing) and I-485 filed concurrently on June 2017.

Company B [Client] decided to get rid of all the vendors by next month and offering full time position to all the contractors.Ready to transfer H1b.

Dilemma:
1) If I should take a FT job with Company B [Client].
2) Just stay with Company A [Consultancy] and look for another Client.

Thank you for your time.

I would just tell the client that you will have ur GC in about 6 months. Ask them if you could just continue for another 6 months as a contractor and join fulltime after that. I am sure they will grant an exception if you are a deserving candidate.

485 wait period is more than 6 months these days. AC21 will allow you to change jobs after 6 months. Instead of restarting the process convince employer B to wait for 4 more months in your case. Make sure employer B job deskriptions are same or similar to PERM application filed by A.