I have never been to Disney before Tokyo, but I can say that DisneySea is my favorite!
Its definitely the best Disney for adults! Its lovely to go with friends and if you use fast passes smartly there is no much waiting in lines!
I also love the Disney goods on sale! Very cute!
And Japan is the best country to wear those goods even outside Disney!
I love how much work they put into all the details of this park. With a friend I spend a fun day here riding as many fun attractions as possible. It's a little bit pricy, but you won't regret the money.
Annotation: can be depressing seeing all the couples while being single ;)
I went here with my wife and we both had an amazing day at Disney Sea!
Unlike Disneyland, which is mostly aimed at families, I felt Disney Sea was more targeted towards adults - in particular couples going on a cool date, or a group of friends just enjoying the day together. In this regard, it felt more relaxed and casual than my previous Disney theme park experiences. They even serve beer at many of the restaurants in the park! Not that children can't enjoy the park, its just that the overall feeling was the park was meant for adults to have fun too! In fact, all of the park is child-friendly (apart from the beer) and they also have designated children's themes such as Ariel's undersea playground.
What you also get is a distinctly different feel from Disneyland in that many of the rides and themed areas are unique to Disney Sea. Tokyo Disneyland, on the other hand, has many of the same rides and is much more akin to Disneyland in the US. I would recommend both if you have never been to a Disneyland before. If you have, then probably just stick with Disney Sea, as it is quite a bit different.
A few words of advice though:
1. Try to come during a weekday as weekends/holidays are way too busy. We came during the week and even then we though it was busy.
2. Arrive early in the morning and get your first fastpass right away - like literally right after you enter the gates. You can only get one fastpass per X# of hours, so its best to get it early to avoid waiting in line for those popular rides. Btw we found the Toy Story ride to be the longest wait.
3. If you are a cheapo, there are discounted rates after 3:00pm and after 6:00pm, but keep in mind these are also the busiest times.