想（= 想要）〉to want to, to wish to. You must be willing, but might not decide or have a certain plan. 要 〉to be going to（= 会、将会、将要）, to need/have to（= 必须、必须要） It's decided, but you might not be willing. 我今年想去上海旅行。 I want to travel to Shanghai this year. (only if I can, because I like to do) (not mentioned if it is already a decision) 我今年要去上海旅行。 I'm going to travel to Shanghai this year. (It is the decision/plan for know.) (not mentioned if I am glad to do) so somethings they are alternative, sometimes they are not. if depends to the context, the situation and the focus. ; )

要 = will (strong intention) 想 = want (weak intention)

要 refers to a decison, or a strong wish 想 refers to an idea 我今年要去上海旅行 means Shanghai Travel has already been on my list and I will go some day this year. 我今年想去上海旅行 means I would like to go traveling in Shanghai, but it depends on the time and money. If have both, then go; if not, let it go.