I personally prefer Tokyo Disneysea, it has more rides and attractions for older kids. But if you have younger children (6 and under), then you may enjoy Tokyo Disneyland more.
depends on what you want from the experience. If you want to buy a bunch of cute Disney stuff, then go Tokyo Disneyland. But the park and rides are the same as the ones in US, just that the ride controllers speak Japanese. But if you want different rides and something different from the the other parks, then Tokyo DisneySea.
I have been to Disneyworld, Disneyland, and Tokyo Disney /DisneySea on several occasions (I actual had a season pass to Disneyland, California for several years and went there on a regular occasions). I would say if you been to Disneyland or Disney World, skip Tokyo Disneyland. It is the same as the American version, but with wider street. DisneySea has a few of the U.S. rides, but it is truly unique and different then the other Disney Parks.