I decided to learn embedded programming for the ARM and my weapon of choice is ARM Discovery Board STM32F4. Ordered one from an internet auction and awaiting.
This board is very simple, yet very powerful. Open source tool-chain support is an advantage. Many people write programs for this board and on the Internet there is a lot of information.
I have also a Renesas Promotional Kit, which I got at Embedded World 2012 exhibition. It is also a small development board, but the tools are most proprietary and limit in use by time or compilation size. Also finding information when something went wrong is difficult, because the number of people using this board is small and I was the first to encounter a particular problem.