I suggest try before you buy.

GameStudio, ShiVa, Unity has free edition to try. You shall take a look at their tutorials/books.

I tried ShiVa and Unity in my leisure time. Among the 2 I prefer ShiVa because you got all features of their free version, though you can't publish.

GameStudio and Unity give you a trimmed version of their engine, although you can publish, however you can't try the 'pro' features.

I just feel more and more GameStudio is target for student for learning a programming language. They limited on Windows platform, no Web publishing, though they plan to port to mobile platform, but which one? Windows Mobile?