It will NOT be chilly. I live in Phoenix and it has been in the 70s lately. Cabo is 850 miles south of here - definitely warmer.
If you have 7 people and you want to book Disney look into 2013, you have to book Disney very far ahead to get a good deal.
Carnival will definitely be cheaper. I only worry that you try a small & older ship.
I really recommend taking the Carnival Splendor. It is the biggest and most fun (for the kids) ship going down there. 7 is a lot of people and you will save a TON by driving.
You can also get rooms for three or four people and save more money. Here is where you need to look: