Swift is still a very immature language, with lots of bugs in the compiler, rough support in the debugger and IDE, and the syntax isn't even set in stone yet (don't expect the syntax to settle down before Swift 2.0, probably some time in late 2015 if not 2016). There are a number of things that you still can't do in Swift (e.g. providing a callback function for APIs that expect a C function pointer), and you'll just spend a lot more time hitting your head against walls than writing working code. On top of this there are many more resources available for learning Objective-C than there are for Swift, and the pitfalls and corner cases are better understood for Objective-C than they are for Swift. As a bonus most of your instincts honed on C will carry over to Objective-C (while they are likely to lead you astray in Swift).
Swift is a really exciting language, and fun to play around with, but it's not ready for production work (yet). It will get there, but in the mean time you should stick with the established tools, which means Objective-C for iOS and Mac OS X app development.