Isn't it a question of what platform so much, as it is what kind of game you want to make? It's much easier and more artistic freedom to use flash over 3dgs, but if you need computing power you don't want flash. Flash and 3dgs both have the advantage that you can produce test environments and prototypes with them really quick and easy, where with UDK you'll spend lots of fun learning the ins and outs of their kit before you can mash up anything reasonably called playable (this is from very short experience with it years ago, forgive me if this is not true). Unity allows publishing heavy games to the web browser, which used to be their main goal. Something yet to be achieved by 3dgs.

So each engine has its advantages and disadvantages for any specific project, but also share many areas with eachother to be compared. An engine is a huge collection of elements, which can be improved by examining/comparing other engines'.

You should choose the engine that suits best to your project. But since people are often too lazy or incapable of learning a different engine than they're used to (I'm no exception), they hope their engine will adapt to their needs, and so start comparing ideas/elements from other engines in the hope it will be developped for their engine too .