Since you're using Mac there are some things to consider here:
* Mac has built in RAW-processing with software that uses the built in Core Image library, so all applications which uses Core Image has the same RAW-processing, uses the same algorithms and therefore has the same results. Only features between the programs can differ. You are already using Preview, this uses Core Image, as well as Apples other image applications.
* Aperture is about to die and to be replaced by the new Photos application that will be released at the same time when Mac OS X 10.10.3 Yosemite is released. It will use ICloud photo library and be compatible with iOS. (iOS already has a version of the new Photos application). Reports from the beta-version shows it is really user friendly with nice power-on-demand for advanced RAM processing. This one will probably be released sometime in march - april this spring, probably at no cost at all.
* There's a new Photoshop contented for Mac, that is being worked upon now and is in beta version: Affinity Photo, from Serif Europe. It looks amazingly good! Very easy to remove parts of image and doing interesting stuff... I'm thrilled! Look at it here:
https://affinity.serif.com/en-gb/photo/
I believe Affinity Photos will use the Core Image RAW-processing algorithms, but you should also be able to use it within the new Photos application: Having Photos as the photo library and then access Affinitity Photos as an external photo editor if you wish. Today's iPhoto and Aperture can already do this...
* Corel AfterShot 2 is a new version of the Bibble Pro RAW-conversion from Bibble Labs (Corel bought Bibble Labs) I used it for some time and I was happy, but then I decided to switch to Aperture which I'm using today... AfterShot 2 is very fast and is very capable, and it has a low price too... But I prefer Aperture, and I will switch to Photos when it comes and I'm probably going to buy Affinity Photos.
I currently uses Pixelmator, it's like a low cost Photoshop 6 but much nicer Macish user interface... This is for when I have the need for advanced image manipulation. For me, Affinity Photos will probably replace it...
Capture One has been mentioned and yes this is very nice in terms of RAW processing, probably the sweetest of them all... I used it when I had Windows and it was wow...
Aperture, Lightroom, Capture One, and the coming Photos, are all photo library software with built-in RAW-processing tools. Photoshop, Pixelmator, Affinity Photos - they are image manipulation programs, they are not primarly RAW-processors. You need both, one library and RAW-processor - and one image manipulation program. THey do different things.