"Red or Blue - which one should I choose?" Try both, figure out what you like. There are a lot of very large companies that use .net There are a whole lot more that use Java. If you're in the Java environment, its easier to go towards Android. If you're in .net, there is almost no barrier to go to all Windows platforms - desktop or mobile. The documentation on the Windows side is (as others have mentioned) truly amazing: MSDN (online or offline). The documentation gaps in either case are filled in by StackOverflow. A career can be built from either one of these technologies. There is no one right answer. All the best.