Tough choice. You have to do both since both are great parks. But since TDS is unique to Japan, and more adult oriented than TDL, with lots of shows and usually a quieter atmosphere....maybe you'd like to try that. Unless you are into the more Disneyfied experience...then do TDL. But you really should try to do both on 2 days.
I concur with TDLFAN. If you have been to one of the other Disneyland parks around the world, try the new and fabulous Tokyo Disney Sea park.
At one time, they had a pass that would allow you to spend the day at TDS and then visit TDL after 6pm (or something like that). You might want to check at the ticket booths to see what your options are.
They do have Pass hopper passes but those are only the 3 and 4 day passports, however, you CAN NOT park hop on the 1st and 2nd days of them so you are forced to do either park on those first two days, then hopp on the 3rd and 4th.
The "After 6 Passport" is almost always available on a daily basis, and sometimes, there is an "After 3 Passport" as well on selected days...
If you're a Winnie the Pooh fan I would, without a doubt, support visiting TDL. The Winnie the Pooh ride they have is simply stunning and is completely unique to any other Disney park. While I wouldn't wait in another half-day line to ride it (I'd be smarter and make a run for it in the morning ), I was extremely impressed with the ride.
But like TDLFAN has said, you need to see both parks. Each has something you won't find elsewhere.