The main aim of this article is to discuss some of technical differences between Flash and Silverlight without any intend to show which among both products is better or worth. This is article does not promote any product and written in rather analytical mean.
Microsoft’s release of Silverlight cross-browser cross-platform plug in, set up a new wave of rumors on the topic of “who’s gonna to rule the Web?” – World’s community of web-designers and developers started to share their points of view towards choosing between Flash and Silverlight. Being a new product, Silverlight finds it hard to get its place on the international market, as it been thoroughly covered by grandpa Adobe Flash. But at the same time Silvelight contains features, everyone have always wanted to see in Flash. The simplest example is search engine optimization. Another interesting fact is that Flash can be developed on either OS X or Windows, while Silverlight requires you to be using Windows to develop any silverlight applications, and not just Windows, but the Visual Studio development kit for Windows which is not in the typical arsenal for web development.
Now let’s have a look on Scripting:
Flash is using Actionscript (object-oriented language with a full range of controls for designing user interfaces) to program Flash objects. It could be integrated with back-end technologies that use other languages and frameworks (e.g. ASP, PHP etc.)
Silvelight applications could be developed using wide range of programming languages, for example Visual C#.Net and Visual Basic.Net, and even client-side scripting with JavaScript.
What’s the difference in Animation?
Flash is using Frame-by-frame animation model, where we create the object for each frame so as to produce an animation sequence. This model is common for traditional animation. The only difficulty could be is that you might need to calculate the number of frames to make a certain time-length animation regarding frame rate per sec. For example you are asked to make four seconds animation with the frame rate 8 frames per second. Thus you need to make 42 frames (4sec × 8 sec/frame). Caught you! 32 frames! 
Silverlight
Silverlight is based on the Windows Presentation Foundation (WPF) animation model, which is time-based instead of frame-based, so you define the start and end conditions, and makes automatic calculations . All, you need to count out is positions of created objects in different frames.
And what about File size?
As you, know Flash uses compressed format, and text and images are inculcated in the movie, therefore the file size is relatively small.
Silverlight is using XAML for its non-compressed description language, , so the size of a Silverlight component is usually larger.
Video and Audio support:
Flash supports multiple video formats. The latest codec is very high quality, and the bandwidth usage is really nice.
Silverlight embeds the industry-standard VC-1 codec for video, and supports WMV and WMA. Microsoft has also made a free SDK encoder for producing WMA and WMV available for everyone.
We analyzed just a few features to compare this 2 exciting prods. Truly speaking the difference is great, and we can only state that choosing among Flash and Silverlight depends entirely on your requirements. Most of web-designers and web-developers share the same point of view that Silverlight is involved when the programming will be complex and Flash when the graphical side is more complicated.