If a video game comes on a console AND PC, the best run for your money is the PC edition. However, if you do not have the processing and graphical prowess to handle the game on your PC, you will hit some major framerate problems. The 360 version of Dragon Age will not look quite as good as the PC version, but Bioware has shown in the past that they do the best they can with the tools they are provided.
In addition, the PC version will most likely have a better User Interface, however as I said before Bioware has done a great job in the past in making the controls simple and manageable for console versions.