Fallout NV: You spend the entire game solving post war geopolitical issues and working with factions of your choosing, either helping them get along or destroy each other as you either work towards a future for New Vegas that you believe to be the best for the wasteland or for your own personal gain. Almost all sidequests affect the overall story and faction relations. There is no such thing as an essential NPC, if your bullets can reach them they can die.

Fallout 3:

Thanks Todd I hate it.

the only reason why FO3 got higher score is because it came out first and surprised everyone with a fresh new setting and combat mechanics.
NV got lower scores because it was more of the same, and because it had tons of bugs thanks to bethesda rushing the devs

of NV had come first, FO3 would be a 7 at a best