Apple released Swift yesterday at WWDC. It's a high level programming language that is going to make developing iOS apps much easier for new programmers.
I know neither Objective-C or Swift at the moment but want to learn to start building my own apps.
There is lot of talk that the old Objective-C would result in much faster apps, but Apple might retire it over the years.
Should new mobile startups continue to build in the old language for now, but make the jump into Swift?
Asking about development environments is a great way to start a flame / troll war. Choose whatever you feel most comfortable to you based on your personal experiences.
At the highest level (and from a subjective standpoint) Swift breaks tradition from objective-c and feels more like a scripting language - something that web developers can pick up quickly. What that means to you depends on your experience / opinions of C, scripting languages and web development in general.
Whether or not it is more productive / performant depends more on the developer and less on the tool.
Use whatever allows you to iterate faster.
While there is indeed talk about Apple eventually retiring objective-c, that day is far away to worry about now.
Even though Swift is a simpler language than Objective-C to work with, do keep in mind that a majority of the support and resources on the web are for Objective-C at the moment (Q&A on Stackoverflow, etc).