I use both. Here’s my take:
DAZ - Pros: Intuitive user interface, native Genesis support, shorter (in my opinion) learning curve. Cons: native 3Delight (render engine) is much slower than Poser’s Firefly engine. Standalone 3Delight may be faster, depending on your machine, since the free version only allows 2 cores. Requires a plugin to animate. Difficult to add ambient occlusion nodes to surfaces, making UberEnvironment the choice method to get AO effects, which are global and sometimes grainy. No IDL without Lux.
Poser - Pros: Firefly is much faster than Daz’s 3Delight, includes native IDL, more documentation for advanced material settings (node-based) including SSS and AO. Includes native support for volumetric atmosheres (haze). Cons: Interface is… challenging, at least at first. Some components are fairly useless—Face room, hair room, etc. Poser “people” (Alyson, etc.) lack the support and quality of DAZ figures.
There is a significant amount of overlap, where both products provide excellent performance. For example, both have great lighting options, the ability to run 3rd party scripts, render DOF, control render settings, etc.
And before I get jumped on for my statements about render times, I have actually rendered benchmark scenes on my machine and have consistently found a 30-50% difference in render times between DS and Poser, always in Poser’s favor—using DS4.5 Pro and Poser Pro 2012.
I like them both. If DS had quicker renders, I’d use it much more than I do. I’m lukewarm about Lux. IDL in Poser gives me results that approach Lux renders. And I’m less than enthusiastic about non-native render engines, because one usually has to alter material and light settings for the 3rd party engine.