Some stuff will sync in iCloud. Media will not. You have to download what you want. Anything you've already purchased will be available for download for free, and you can download entire albums, artists, etc (ie you don't have to do it song by song). You can also change he store settings on each device so that new music bought on one device will automatically download on the other (and or iTunes).
You can still sync with iTunes when using iCloud; so you could do your initial sync in iTunes to get what you want on the iPad, then tweak it in iCloud as needed later.