That is a very good question. I have been using both programs for years. While i feel that daz studio improved and went further, Poser came to a standstill. Like the comment above, it is much easier to create for studio than it is for poser. Studio has also content creation tools, it is less work. Oftentimes you can import scenery into Poser but you have to correct the shading etc.

Poser has a lot of content but also a lot of it is outdated. They released la femme and l'homme. I personally really dislike those models, strange gaze and also a strange form. In the end they are just polygons and they can be altered. I can create the same morph for genesis in less time because it fits better in my workflow.