I'm not sure that the 727 is worth the investment in time for the benefit that might arise from it, night cargo builds hours slowly and as you point out it is a small and shrinking fleet. If the option is there to build time on the Emb 145 I think that would be of more use to you in the long run, however I suspect that the 727 would be a more satisfying challenge to master.
I speak as one who has never flown an aircraft that was still in production.
Good luck.