Photoshop CS6 is the same as the version in the CS6 Master Collection. So you would only get the Master Collection if you needed Photoshop plus some of the other programs, like AfterFX, Premier, Illustrator, etc.
Photoshop CC2015 is part of the Creative Cloud subscription so if you "buy" this you are really paying a subscription to keep using it. If you stop your subscription then it stops working.
Tools wise they really haven't added anything major between CS6 and CC so really you should decide if you want to buy it outright, and get a version with no updates, or pay a subscription and always have the latest version.
Personally I use the Creative Cloud Photography Package which is Photoshop plus Lightroom and that costs me $12 per month:
http://www.adobe.com/au/creativecloud/photography.html