Two very different games. If you like RPGs and first person shooters then choose Fallout 4 (the mods will be amazing). If you like fighting games go with Assassin's Creed.
Alternatively, you could always get both.
Assassin's Creed: Syndicate for $31.73
(http://cd-keys.us/pc/games/assassins-creed-syndicate-pc-uplay-cd-key.html)
Fallout 4 for $46.99
(http://cd-keys.us/pc/games/fallout-4-pc-cd-key-steam.html)