Casual - because i don't know what realism mode is (die = start again?)
"hardcoe mode is an optional setting which makes the game more realistic, with gameplay elements such as dehydration and healing over time."
"Normal(casual) -> hardcoe(realism)
Stimpaks heal instantly. -> Stimpaks heal over time.
Ammo is weightless. -> Ammo has weight.
No dehydration. No starvation. No sleep deprivation. -> The PC must drink, eat, and have proper sleep cycles to stay alive.
Crippled limbs can be healed with stimpaks. -> Only a doctor or a doctor's bag can heal a crippled limb."
http://fallout.wikia.com/wiki/Fallout:_New_Vegas#hardcoe_mode