Readers,
is it better to buy tickets for Disney before travelling to the US, when in Florida, at ticket shops or at the gate? Where is the best deal?

In general, Disney tickets are not discounted. The only deal I've heard of is through AAA, if you are a member, and that is either 5 or 10%. Also, since I live in New Hampshire, there is no sales tax on the tickets when I purchase them in my state.
That said, the only other benefit to pre-purchase is that you won't have to wait in line when you arrive. And since Disney means a lot of line-waiting, it never hurts to get a jump-start. You can purchase them at Disney stores, and most hotels (concierge desks) in the Orlando/Kissimmee area. Just be sure they are not adding a sur-charge (you can check the ticket prices at the Disney website).
Also, be sure to use Fast Passes in the park to try to minimize waits.
Have fun!

I relied on a website, mousesavers.com when i planned a Disney trip. the site has tons of info which will help you, including the best places to buy tickets.

You can prepurchase tickets on waltdisneyworld.com for a discount. They are then mailed to you. I think it's a 10-15% discount to prepurchase.

In my experience the best place to buy Disney tickets is from Disney. The best way to avoid lines while there is to use FastPass whenever possible.