I just asked this very question with a Disney Cast Member on the phone. One major thing I discovered is that if Disney World happens to be running free dining or any other offer, you won't get the offer if you book land and sea package.
It is best to do them each on their own to get the best deal.
Easy way to figure it out is get on their website and do the math with each one.
Stay away from getting air
tickets with Disney... much more expensive usually. I just did this last week and saved about 600.00 getting it on my own.