I just got an a7s and opted to stay native with it, but my reasoning may be different than others'. I plan on remaining in the Sony ecosystem (a7s/r-fs7-f5 etc) so I had no reason other than price to stray from using Sony glass. The reason you would want to use an adapter is that Canon has more affordable options than Sony does ATM (although I'm predicting Sony will come out with a full midrange FE lineup in the next 2 years, and Sigma/Tokina/Tamron will come out with alternatives soon as well), but you also have to remember that you need to put up the initial $400 for the metabones before you can take advantage of the price difference.
Another reason I went native is that autofocus is worsened and slowed down when using an adapter. It's a slight difference, but noticeable especially when you're trying to shoot sports or portraits. The connection can be faulty and there's always a greater opportunity for issues to arise when you add another piece of technology to your setup.
Lastly, the great thing about the FE Zeiss glass is that it is absolutely unbeatable in terms of optical quality. I have the 24-70 F4 and have shot wiith both the 35mm f2.8 and 55mm f1.8, and let me tell you, the color accuracy and sharpness I saw was crazy good. They're fucking expensive lenses, but you'll NEVER have to replace them for something better unless you're jumping to cines or Otuses, but that's when you're a working professional and price isn't an issue.
So yeah, make your own decision, cause you probably are looking for different things than I am. You can't really go wrong either way but there are definite pros and cons.If you've already got GOOD Canon glass (don't buy a metabones to use your nifty fifty) then by all means go with the adapter. Otherwise, think about your intended uses and make the call.
