In CSP I can have fx 15 pages open in 15 tabs in the same time.
What advantage would I have in EX if I there had the 15 pages open at the same time?
The advantage with EX is that there is a page manager. You don't need to have 15 tabs open at the same time, you can just open one story file that contains the 15 pages (or more - up to you) and the page manager opens with all 15 pages in small thumbnails. You can open pages and close them, open one or more and flip through them. Everything is controlled through the page manager. You can export your pages in a batch process (instead of exporting one page at time with the Pro version). You can even apply actions and more to all of the pages in a batch process instead of one page at a time.
If you're working on a large project and considering going with EX, you should do so early in the project rather than waiting too long. Importing pages into a story file is a slow process and if you're working on a 120 page graphic novel or something and you get to page 50 and decide that you want to take advantage of the page manager in EX, you'll be stuck importing all of the pages. I don't think there's a fast way of doing it aside from going one at a time.