Are you sure Poser does HDRI?  It's one thing to do IBL (Image Based Lighting), it's another if the image types supported include images with 16-bit per channel or better, which is where HDRI resides.  IBL with 24-bit images is great too, but the results are better with high dynamic range images. Better in this contact means more realistic, or closer to real lighting. Sometimes realistic is not the better way, because there's a cost in rendering time and memory sizes etc...

IBL is something that was already in TrueSpace 6.6 and 5.2 , what, 5-6 years ago?

Of course, it's one thing to have a feature, it's another thing to make it a no-brainer for usability. Carrara is by far easier than TrueSpace to set up and enable IBL, indirect lighting (radiosity) and caustics.

I think if you do a good and thorough test of demo versions and analysis of the results you'll find that Carrara is the better of all these for rendering HDRI and IBL and caustics and radiosity all together. In terms of quality of the render, in terms of speed too.

But it's always a function of cost too, so if Carrara 5.1 is out of reach check and compare with Carrara 4 or go with Bryce. Or both.