Our company has just gone through the exercise of creating an OpenGL engine (as part of a productivity tool)...we used OpenGL due to the fact that we wanted to be able to port the engine to Mac. This worked well until we discovered that most Windows PCs around today have VERY poor OpenGL implementations.
DirectX is kept up-to-date with Windows Updates so windows users generally have good DirectX releases on their machines. Motherboard manufacturers have an interest in keeping their integrated graphics cards DX compatible where as many (mainly integrated graphics cards) have only basic (and buggy) OpenGL drivers for their moterboards.
We have just about finished a DirectX version of our engine to help users who cant get the engine working using the OpenGL version. The Mac uses OpenGL exclusively so we had no problems there

This should be taken into account when deciding on the 'best' format to use.
Thanks for the great tutorials and I am looking forward to the XNA release. I am trying to get our company to start looking seriously at development in C# and Managed DX ... wish me luck
