I think the source materials are pretty much the same but in Korea it's prepared a little differently. The biggest difference is that amount of sauces and extras. For example, the filet o fish in the US is usually drowning in tartar sauce but in Korea it's usually just spread on top (like mayo).

The big macs are a little neater and there is usually less lettuce and secret sauce.

Of course everyone is free to choose where he/she eats and I don't want to be too preachy but I stopped eating McDonalds about 3 years ago mainly due to the following:

1) Everyone knows that the food isn't good for you but most people don't know how bad it is for you. Even 1 meal a week is enough to really wreak havoc to your internal systems (higher cholesterol, liver functions, etc.). Most people think a meal a week or a meal every other week is fine...it's not.

2) There are so many chemicals and preservatives that it only bears a visual resemblance to food. Did you know that if you buy a hamburger and lay it down on your desk...a year later it will still be there without any mold or rot? It will simply dry out. Sorry but that's not how real food should react in this world.

And there's obviously much more...