I have just finished learning application development on android platform, as In matter of language it was easier to understand. However the convention drove me crazy yet simple enough to understand. Now the dillema is I am about to jump in iOS development. Language part wont be hard to understand but I am bit worried about design pattern or use of Xcode.
What challenges I might face which differentiate these two develoment enviroment? And which one is easier to deal with and more productive?
See the "Java" section in this post. Ease and productivity are in the eye of the beholder. Many find Objective C to be incredibly natural and beautiful. Others find it bizarre and frustrating. If you're willing to embrace the iOS way of doing things, you can pick it up very fast. If you fight it and try to keep coding in Java ways (or C way, or C++ ways), then it will be very challenging. The trick is to really work through the tutorials and not just try to figure out "how do I do X that I'm used to doing?" Often the answer is "you don't do X. X happens automatically when you do Y." But the place to start is working through some tutorials.
The Apple documentation is mostly excellent. You should spend time reading through the Programming Guides. Don't just jump to the References. You'll be very confused.