Want to have a bit more space between my butt and handlebars. The options are longer stem and setback seatpost. What are theoretical advantages/disadvantages of both options?

When pushing, I instinctively try to move back in the saddle, so seatpost seems to be better option for me. Current setup - Trek Emonda ALR 4 54cm with 90mm stem.

P.S. I already moved the saddle all the way back.