One of your questions is easy to answer. Buy your tickets from the link above for Orlando Park Tickets, great value for money and so easy to use. You get the actual tickets not vouchers which you have to queue to excahnge and you can get them delivered in the UK, US or pick up in the US.

Your other question Universal or Disney is harder to answer. As we go most years we do have times when we don't visit Universal or Disney but we know we will do it the following year. Universal only has the one water park, unless you are thinking of getting the Orlando Flex ticket which also includes Sea world and Aquatica. I like the fact that you are not pushing yourself to do too much in the 2 weeks, it is nice to have some time relaxing and shopping.

You also need to think about yourselves, I personnally can't take too much of the big roller coasters at Universal, but can handle most of the disney rides, my stomachs tolerance seems to get worse with age.

There is also a much wider variety with Disney as you have 4 parks rather than 2. You could get the 5 day ticket which includes athe water parks and some other options.

With the 9 year old son it could also be worth considering a visit to Kennedy Space center.