I already have it on PS3, but I will 100% purchase it on PC. According to what I've read, your GTA Online character will transfer across versions, so my PS3 online character with his apartment and everything can now be on PC, with the additional wildlife and player count and graphics that are sure to accompany.
I honestly was excited to get Watch_Dogs, but man that game did not pan out to the hype. It is definitely not a huge graphical improvement over GTA, with an extremely shallow city, mechanics, and story that I honestly am excited to get back into an open world game that knows what its doing. Considering how amazing the Max Payne 3 port was, I'd also wager GTA V might be better optimized than Watch_Dogs.
I only hope the amount of content addition is significant and that the price point is around $50 or less.