I have the original AP in physical and pdf form. I'm considering running this AP. However, I'll likely be converting it to 5E D&D. If I were going to run it in Pathfinder, I'd definitely spring for the new version, but since I'd be running it in 5e, I'm wondering if people think it's worth it.
Apart from the new Pathfinder stats, changes are mostly minor. I bought the Hardcover anyway because I'm such a big fan of this AP, so if you want to spend your money for that reason, feel free. But there is no real need to buy the book for the 'extra story elements', because you might be disappointed. You'd better spend your money on modules that you can insert in the AP, like 'The Harrowing', 'Academy of Secrets' or possibly even 'House on Hook Street' or 'Conquest of Bloodsworn Vale'.
If you're looking for ideas to change the story to your style of GM'ing, I'd recommend searching these boards and some AP journals for alternative storylines. There's a lot of great material out there.
There's a new dungeon and a few new characters (like Vavana and Kordaitra), but for the most part it's the same adventure. If you're going to be converting it to a different system anyway, then it doesn't really seem necessary to get the anniversary edition.
I would say that Scarwall has about three times more encounters in the hardcover.