Since you got the season pass you'll get all the DLC. Basically you get the DLC right when it comes out on your console. I've had the season pass for MW3, BO2 and soon I'll get the season pass for Ghosts. Is there some type of benefit if you pre-order Onslaught? I'm pretty sure if there is there just trying to amp you up for the new DLC release. It's a marketing scheme to get your money ASAP. Don't get played like a suka!
Yeah Ive never seen nor heard of a benefit coming out if pre ordering a map pack.
By buying the Season Pass you get all four map packs that will come out for Ghosts for free. In effect the Season Pass is a pre-oreder for all the map packs. The advantage of getting the season pass is about a $10(depends on your currency) reduction in price compared to buying all four map packs individually. Season pass also gives you the Team Leader Pack (patch, background, etc) as well as the Free Fall Map.
In saying all of that you still need to go to your store to download specific content such as the free fall map. When Onslaught comes out for Playstation (Feb 27th) all you need to do is to go to the store and download the map pack...for free.