You can almost always get a better deal by booking everything separately. No need to pay for "extras" that you probably don't need or want anyway. And it isn't hard or complicated. Mousesavers.com helped me tremendously. Not only do they keep us informed about discount codes and such, but there is a lot of great planning advice.
As for off-site or on, I think on-site is worth it even if it did cost $150 more. It is so nice to be able to park your car and not have to drive for the duration of your trip. Or if you do choose to drive to the parks, your parking is free anywhere on Disney property.
On-site guests also get the benefit of Extra Magic Hour, where a different park opens an hour early each day for resort guests only. You'd be amazed at how much you can get done in that hour! And one of my favorite perks is that when you shop in the parks, you can have your packages sent back to your resort to pick up the next day - no carrying those bags around all day or trying to do all your shopping at the end of the day when the shops are packed!
Do some research and find out what is best for your family. Check out everything you can on these boards and ask lots of questions. Don't limit yourself to the advice of a
travel agent; I have found the people here to be much more knowledgeable on the subject of WDW, due to their own personal experience.
Have fun planning - it will be worth it!