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.
; )