I myself would simply go with both, but I am tight on money, sadly. I am a 16 year old who has worked and saved for my money, haven't asked my parents for a single dime. If I did have the money, I would definitely get watchdogs, but I am going to BF4 as I like shooter games and I have never played BF online before.
"Watch Dogs" for me. I've never been to deep into multiplayer,so the single player experience is what makes me decide between this games. No doubt BF4 campaing is going to be amazing, but not too long. "Watch Dogs" seems to be much larger and replayable.
I'm never getting a Battlefield game again. When people have their own servers, they abuse the "power". I've been kicked from servers plenty of times just for killing the admin. It's ridiculous. No more Battlefield. Ever.