Hi Julie - and welcome to the UK DIS!
This is a tricky one - and I hope you get some more answers here later but if it were me, I would try to decide if this was a one off trip or - if there was a likelihood of a return at some time in the future.
I have just had a quick look round for you and if you want to do two days at BG and 1 at SW then check out the Busch Gardens and Sea World 2002 Fun Card. It gives you unlimited use of the two parks until December - with no black out days - and only costs $94.90. The only downside is the offer is available only until May 31st so you would have to decide pretty quick if this is suitable for you. The details can be found here -
BG & SW 2002 info -
and you can buy online, and print out a receipt/ticket which you take to the park, insert in a special machine and it gives you your ticket. No queuing
A single ticket to either park is $44.95 - so basically you'll get one day free, and the option to use it time and time again
I think I have interpreted the information correctly - but I would make sure you read all the terms and conditions on the site
With the Disney tickets - the single price is $44.95 for the parks and $29.95 for the Water parks - unless you intend to go back fairly soon - then a single entry ticket would be the best option - but of course, this would not give you the opportunity to Hop from park to park should you decide you wanted to do this once your there.
Difficult choice - but the BG/SW does seem a great offer at the moment.
HTH