What are the advantages of LLDB over GDB in iOS development?

The most notable advantage is that LLDB understands dot syntax in properties:

po self.property

A quote from LLVM project blog:

LLDB supports basic command line debugging scenarios on the Mac, is scriptable, and has great support for multithreaded debugging. LLDB is already much faster than GDB when debugging large programs, and has the promise to provide a much better user experience (particularly for C++ programmers). We are excited to see the new platforms, new features, and enhancements that the broader LLVM community is interested in.

Another quote from LLDB homepage:

LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser and LLVM disassembler.

Why a new debugger
In order to achieve our goals we decided to start with a fresh architecture that would support modern multi-threaded programs, handle debugging symbols in an efficient manner, use compiler based code knowledge and have plug-in support for functionality and extensions. Additionally we want the debugger capabilities to be available to other analysis tools, be they scripts or compiled programs, without requiring them to be GPL.