well i have used both Poser 5 and various versions of Studio and i find Studio far more intuitive to use. to me the Poser interface is very clunky and counterintuitive. the thing that is missing from Studio is a useful dynamic cloth facility. the Optitex dynamic cloth may be technically superior, but the files are huge and the cloth cannot be scaled.
(the Poser set-up is superior in that scaling seems to work.
plus you can usually take a conforming cloth into the cloth room and convert it to dynamic.)

plus Studio3 Advanced is already fully 64bit ( and not just the render engine )and only a few plug-ins are yet to be brought over. it has GI, IDL , comes with an HDR image converter for use in lightdomes ( IBL).
has a 'material room' ( shader mixer and shader builder ) i have no experience in these as yet, but by all accounts a lot of content developers have said it is easier to use than the Poser Material Room.
and with lights you can look through them to see exactly where they are aimed.