Courier would win.
Enclave power armour, lots of healing stuff, lots of drugs to boost DT, abilities, and strength, Esther the shielded fatman, many mininukes and varities, all the weapons, grenades, dynamite, throwing spears, VATS.
And they could always go to the Divide and drop massive nukes on them.
They could also use animal friend to have rad mutant animals fight for them. Also have Mysterious Stranger or Miss Fortune. Also could use the NCR radio to call a NCR Ranger.
Dovahkin has light or heavy armour or robes, melee weapons, archery, magic, shouts and racial abilities. But nothing to boost DT. Arrows won't do anything against power armour.
Dovahkin could use Call Dragon and Kynes Call shout to get animals on their side, but it won't do a lot against someone who's able to fight deathclaws.
Or they could just be friends and talk, if they were both peaceful and calm.
Then if they fell out, fight.
Or if both were psychotic pranksters, do some evil reverse pickpocketing.
Grenades do more damage than poison and are more messy.
And there's always stimpacks for poison fighting.