OK, how to say this without too many spoilers. ;)
It sounds like you've completed the first quest line:
That's the four (or five, if you're fussy) Ages, collecting the journey cloths in the shape of the hand, ending with returning the pillars.
That's the end of the "original" content.
To D'ni adds
most of the core Cavern and the Nexus, along with some side areas from existing Ages (the Baron's Office, for example). You'll have a Nexus book in Relto to start that area of the content.
Path of the Shell gives you
a second "hub", and a new quest. You start that from the Watcher's Sanctuary (again, you'll have a book for that in Relto).
The original content is exactly the same between Chronicles and Myst Online. Most of the To D'ni content is the same (I believe it's a little easier to get around in Online). Path of the Shell is mostly the same, but with a different ending.
And of course, Myst Online has more content that isn't in Chronicles (stuff added after launch).
I'd recommend just playing through everything on Myst Online. It might be worth running through Path of the Shell on Chronicles (just for the different content).