We have been looking into this over the last couple of days and have decided that for us, the best option financially is to pay when we get there.
We plan to buy a 7 day ticket for Disney and will also go to Typhoon Lagoon at some point. A 7 day hopper with 7 entries to the water parks/Downtown Disney attractions worked out £10 more than what we actually need (if purchased in the UK).
The Orlando Flexticket is not too bad on value if you buy it there as that allows unlimited access to the Universal and Busch attractions as well as WetnWild.
Overall, I am pretty sure that the price is very slightly cheaper on the gate. The UK options are for more days than you actually will use, so not worth paying the extra (unless you want to be in a park every single day).