I found ciV to be more historically immersive for me.
Well, obviously the hex tiles make the world as well as my empire look much more realistic.
The fact that most of the land is not occupied by civilization during early ages and that there's much vacant space even during 1600AD or so seem more historically correct.
The fact that militaristic conflict or at least enough force to expel one is unavoidable is also more historically immersive. I don't expect an empire full of science, culture and all that juice with no defense to be untouched by warmongering neighbors.
I also think city-states are there to help the world feel more realistic. It was over simplification to have 8 countries and babarian cities to defict the history of the world. Now that we have 8 civs along with 12 city-states the world feels more organic for me.
To be fair there are aspects of Civ5 that do increase historical immersion. In prior Civs, there was definitely way too much early overexpansion and occupying every tile by 1AD. In that sense Civ5 is better. Also quantified resources is clearly better. Also lots of city-states amongst major powers is in and of itself not bad and definitely historically immersive. Its just that they are way too overpowered in terms of benefits and overly crude in that you just buy them off. So these city-states resemble like nothing in real world history.
But on net balance, Civ5 removes much more historical immersion than it adds. If they had kept all historically immersive elements from Civ4 (even if streamlining it or changing it somewhat) and added well implemented but good stuff that would have been great. But they added little and removed a lot that was good and replaced it with junk (like global happiness mechanic, science=population, etc etc) that are just way crude and flawed and largely destroyed historical immersion.