Zika: New stuff, no one knows enough. There's speculations that it is related to the recent syndrom of newborn babies being born with underdeveloped skulls, and chances are there is a link between the two. However, it's not been 100% confirmed the babies are born like this because of zika. Chances are they are. But there could be other underlying causes, such as contamination etc.
Dengue: Old stuff. We know for sure it's transmitted by mosquitoes, what it does and that we can die of it, although not necessarily.
Bottom line: It's all here, right now, in Buenos Aires. We have plenty of mosquitoes here. Neither of the two is an STD. All it takes is a mosquito bite. We have absolutely no idea what zika does to adults who are not pregnant. It might affect some part of metabolism, it may play a part in developing auto-immune issues. No one knows. Plus, we have absolutely no control over if the mosquito who bites us has the virus or not.
Personally, I'm not afraid of that.