There is no correct answer. It really depends. If you use Swift like a different syntax for Objective-C, it's probably way faster to keep writing ObjC (because of the broken tooling).
If you can leverage other patterns, such as FP, you might be able to achieve a big speedup in writing Swift.
It also really depends on when you want to ship: if it's a bit more long-term, Swift might be a better bet (I'm assuming that in a few years from now, almost all iOS apps will be written in Swift). If it's more important that it's ready next month, ObjC might be better, because the devs will know exactly what it takes to ship it, and will almost certainly not run into unexpected issues with the language or compiler.
That said, I'm writing my next product 100% in Swift, and we're on schedule to ship in 2 weeks. I wouldn't have done it any other way, and have seen a big increase in productivity and fun.
If you can leverage other patterns, such as FP, you might be able to achieve a big speedup in writing Swift.
It also really depends on when you want to ship: if it's a bit more long-term, Swift might be a better bet (I'm assuming that in a few years from now, almost all iOS apps will be written in Swift). If it's more important that it's ready next month, ObjC might be better, because the devs will know exactly what it takes to ship it, and will almost certainly not run into unexpected issues with the language or compiler.
That said, I'm writing my next product 100% in Swift, and we're on schedule to ship in 2 weeks. I wouldn't have done it any other way, and have seen a big increase in productivity and fun.