I've been testing all 3 on my Macbook Air and older iMac. I have Safari set as my default but use all 3 about equally... just because I can
Safari 6.0.5: Pros: Definitely the best OS X integration. Uses system spell-check, auto-complete, dictionary, and gestures. Fastest scrolling using TheVerge as a test site. While generally #2 in benchmark testing, Safari is the fastest at Flash and Quicktime throughput, especially when streaming live feeds from CNN, FoxNews, and I've been watching the fire coverage on KKTV.com.
Cons: The thing I hate most about Safari is the wide tabs. I use Glims to fix that and add favicons to them. I also hate how Safari reloads pages when swiping back, but doesn't if you use the back arrow. On my iMac running 10.7 Lion, I have to force Safari to sync bookmarks while my MBA 10.8 Mountain Lion and iOS6 devices sync without problems. Safari generally behaves itself memory wise on the 2Gb iMac but on my 4Gb MBA, it will quickly eat up to 1Gb RAM and easily get to 1Gb of swap file. But, it's never crashed on either machine due to low memory.
Chrome 27: Pros: Better interface than Safari, better tab management, built-in task manager, and supports some OS X services like gestures and dictionary. Has it's own spell-check but doesn't auto-complete. Best built-in PDF support of the 3. Better search features from the omnibar as long as you use Google. Seems to be better optimized for Google services like search, YouTube, Gmail, Maps, and Google Now. The voice search does work very well. Lots of add-ons and themes available but still falls short of Firefox for extension support. Chrome's built-in PepperFlash is just a tad faster than Adobe's Flash plug-in, but also uses more threads and CPU cycles and doesn't have the throughput of Safari.
Cons: The separate process model Chrome uses makes it the most memory and CPU intensive of the 3. The more tabs and plug-ins you use, the more memory it will eat, but it releases memory better than Safari or Firefox when you close tabs. Chrome also wants to use the GPU for everything, driving up temps 20-30 degrees on my MBA. Chrome is also 32Bit so it doesn't support Java 7.
Firefox 21: Pros: Arguably the best interface, or at least it's my favorite. This will change when version 25 is released and Mozilla implements the Australus theme. Excellent tab and bookmark management. Extensions, extensions, extensions. Best memory management of the 3. Better PDF viewer than Safari (when it works). Best developer tools built-in and FireBug is a killer extension.
Cons: Consistently and measurable the slowest of the 3 but it's the least prone to Flash crashes, at least in my testing. Will sometimes hang on heavy javascript pages and auto-updates pages like Facebook and WUnderground's animated radar, especially when in background, like the front tab takes priority. The Gecko engine also doesn't render as accurately as Safari and Chrome which use Webkit. I really had no idea of the rendering differences until I ran them side by side. No print preview.