You listed four "requirements":
1- Side-mounted paging buttons.
2- Touch-screen note-taking.
3- Under $200 price.
4- cross-platform sync
Unless something changes drastically with the upcoming Sony and Pocketbook intro's, you'll likely have to pick two or three out of the four.
Side-mounted controls and touchscreen note-taking/highlighting, etc is doable with the Boox and Pocketbook 302, among others. But not under $200. And no crossplatform sync.
If you give up on the ergonomics and the sync, the Sony might do it.
Give up on the touchscreen and Kindle and Nook do everything else.
I'd say wait to see what Sony and Pocketbook are up to; shouldn't be more than a couple weeks.
In the meantime, consider *where* you're going to get your DRM'ed ebooks and how much value you expect to get out of the touchscreen, sync, and annotation capabilities.