Learning to develop Android Applications in Java is going to give you some basics in Java development. But your primary platform is mobile device so it is not going to give you any knowledge of how to use Java on a backend server. Even though the language is the same the problem space is different.
Thus learning to develop on the iPhone using objective-C is going to give you some experience with Objective-C and OO in general. It is not going to make you a MAC application developer or an OO developer. Though you will be able to apply your knowledge there will be a whole bunch of new stuff that you will need to master.
In both cases what you learn will help you by giving you experience in general. Neither is going to lock you in but then neither is going to teach you everything you need for a particular language or problem space. There will always be more to learn and new techniques to master.
In your next IPhone project build the back-end in C++ just to give yourself the opportunity of learning a new language (while still remaining within your comfort zone). Then each new project learn something new.