Coming from a IS300h, with a GS300h you have same power , so if you think it's enough your IS it will be ok GS 300h too.
About extra costs, GS450h it's obviously more expensive, but the v6 power it's on another planet, I had a 2008 GS450h and when i bought a 2014 GS300h I missed that V6, even if in normal driving the four banger is more than adequate, and less thirsty (from 12,4 km/l of v6 to 16 km/l my average).
Quality and refinement in GS is far better , even more if you choose a premier version.
Space for the driver? I don't know, but seats with 18!! settings with memory are very comfortable.
Now I drive a RX450h mainly for 2 reasons: I missed v6 and wanted to try a SUV, it's fast but compared to GS 450h it is "quiet".