Ifyoucanactuallysparethetimeandthemoneyforit, then I wouldrecommendthatyoutrygoingtobothparks, especiallyifthisisyourfirsttimeinthe "HappiestPlaceOnEarth"
However, ifyouhavebeentootherDisneyparksintheworld, TokyoDisneylandmightnotofferyou a completelynewexperiencebecauseitdoessharesomesimilarattractions, suchas "BigThunderMountain", withotherDisneyparks.
Ontheotherhand, there's onlyoneDisneySeaintheworld, anditoffers a differentexperiencealtogether.
Soifyouhadtochoose, I wouldprobablyprioritizeDisneySeaoverDisneyland.