Single world. There are different regions but they are seamlessly connected. The only loading screens are for certain cutscenes -- probably because they will use higher-polygon faces like in Xenoblade.

The best comparison for the first Xenoblade is any tradional MMORPG game. You have "zones", but they are not separated by loading screens. Instead you have narrow passages which act like small zones (like caves or paths or a bionis' stomach haha) in between the bigger zones which feel organic, and they're diverse transitions. Now this worked for Xenoblade Chronicles because you are going from the calf of a bionis linearly to its head and then back down to its sword and then across to the mechonis. You are basically moving in a single direction up, down, and across at any point in time (although each area is so vast that you can explore within it.)

From what we've seen of Xenoblade Chronicles X it is going to be more of a traditional world (not two giant humanoids.) For that reason it seems more likely that you'll have a more 2 dimensional (3 dimensional with mountains and floating islands) choice of direction. Furthermore, from what we've seen of the gameplay trailer there will be multiple ways to bypass certain landmarks, and probably multiple ways to enter different environments, like Zelda U. I expect it to be as seamless as an MMORPG. It is not a sandbox game, however. So don't expect to go anywhere you want at any time in the story. Although, who knows. They are obviously trying to make this game less linear than Xenoblade Chronicles. Maybe you can go anywhere you want whenver you want. 

Different zones in Xenoblade are separated by loading screens. Not so in Xenoblade X.

"Xenoblade was traditional RPG, whereas Xenoblade X is an open world RPG. Xenoblade was linear despite having big world. In Xenoblade X, you will get quests from different places and have more freedom."

It does sound like you can go anywhere you want whenever you want. Sure there may be certain times where specific areas are inaccessible, but I think for the most part the whole world will open up early on and from that point you'll be able to freely explore it.

Here are the 8 unions, who will you join!!!!

I will join all of them and do all the quests and save all the cats

You can't complete all of the quests without saving all of the cats! XD

Testament.

Nevermind

Hahaha. There will probably only be a couple of cats. Probably.

No one said you have to be nice to the cats after you save them, either.

Yeah I never really considered the transitions as, "loading screens"  and forgot all about them. Even still they were like 3-5 seconds long and had a moving frame, versus say a loading screen in Skyrim's cities or dungeons which were about 10-30 seconds long and had one or two still frames. 

As for Xenoblade X, I expect there to be level walls which will prevent players from traversing certain zones (they'll be killed.) Still I think there will be actual inaccessible areas.