Xmarks is completely different. You can uninstall xmarks and just use Firefox sync if you want.

Xmarks is a separate service operated by a different company, now part of LastPass. More info: http://www.xmarks.com/about/company

The advantage of Xmarks, assuming it works, is that it syncs with browsers other than Firefox, too.

I am not familiar with this issue of a possible conflict between Xmarks and Firefox Sync. Hopefully a Sync expert can help you sort that out.

What we need to know is If we use Sync will all of the data saved and imported via xmarks remain, or will it be LOST?

Using both Xmarks and Sync at the same time doesn't work out, as reported here by many users who have tried.

Once you disable the Xmarks add-on, your bookmarks will remain in Firefox. But if you were using the Xmarks "tags" feature your "tags" don't appear in Firefox. Xmarks "tags" aren't compatible with Firefox's tag system [from what Xmarks users have said]. But Sync will use the Firefox "tags" once you get them setup in Firefox.

No reason to change from xmarks then, more trouble than it's worth.

which one works better ? xmarks sync or sync in firefox sync?

It's all personal preference :)