Hi,
both results are correct. Wolfram Alpha's step-by-step solution gives:
![enter image description here](http://community.wolfram.com//c/portal/getImageAttachment?filename=ScreenShot2016-01-16at01.05.02.png&userId=48754)
In the step-by-step solution it says "Simplify the expression...". It is easy to see that the expression simplifies to:
Simplify[-(x^2/(2 + x^2)^(3/2)) + 1/Sqrt[2 + x^2]]
(*2/(2 + x^2)^(3/2)*)
or
![enter image description here](http://community.wolfram.com//c/portal/getImageAttachment?filename=ScreenShot2016-01-16at01.08.05.png&userId=48754)
You can obtain that in Wolfram Alpha, too.
Cheers,
Marco