Two totally different games. Do you want a predominantly online multiplayer shooter or do you want a predominantly single-player, open-world action-adventure game?
Are you a BF fan? I'm not, so Watch Dogs would be my choice.
However, if you want something that's maybe in the middle of those two, look at Tom Clancy's The Division.
Did you not see the epic BF4 video? Did you not see the tank fall into the subway and the sky scraper come crashing down??? How can you watch that video and not say... HOLY CRAP (crap being substituted for real word) HOLY CRAP I must go and blow that building up. The game looked like it packed hours and days and weeks and months and years worth of fun. ....
Watchdogs looked cool.