Hmmm.. a bit weird. Both tools were heavy competitors in the late eighties, before being acquired by their present owners. But Adobe continues to invest (somewhat) in PageMaker also hoping to migrate customers to their more complex and more strategic suites, while for Corel Ventura seems to be no more than a small cashcow. Anyhow, seeing the profitability of Corel one could really wonder whether it makes sense to move more towards their products...
So far history and strategy. As far as I recall from functionality, they are quite different beasts - with Ventura more for the book type of document with many repeating patterns, and Pagemaker more for the flyer type of document. They must have blended a bit since then, but the basic architecture will always remain...
Have fun,
- The Netherlands