Are they really such similar titles? I'm personally quite interested in Watch_Dogs but really couldn't care less about GTA 5. Admittedly, that's mostly because of the setting.
I think it's mostly my own personal patience.
I'll be playing GTA 5 in September and that will last dozens of hours and probably the better part of the month following the game's release.
After that going DIRECTLY into Watch_Dogs in November feels like it's going to be too much open-world gameplay in such a short time.
Of course I could always wait to play Watch_Dogs, but that's not my style, I like to play everything I'm interested in at release to be a part of the launch experience. I find it more enjoyable to play games when everyone is talking about them and the hype is at full capacity.
I'm definitely expecting to experience some serious genre fatigue when Watch_Dogs comes out. Whichever one you may be more excited for, if you're interested in playing them both I think that's definitely a legitimate concern.
This game will be compared to GTA 5, it's inevitable. And it probably won't be good for Watch_Dogs.
EDIT: Oh ya, I just remembered Saint's Row IV comes out in a few months, right before GTA 5. Ya... genre fatigue for sure.