As understand it, 须要 is stronger and more formal. It implies an obligation. So for like contracts you'd by usin 须要, but if you hear someone say xu1yao4 on the street, they probably just mean 需要。

需要 need to 须要 must according to their meanings you can't use them interchangeably and in fact we hardly use 须要 unless in some very formal essays perhaps (that's my guess anyway)

In Taiwan, we think they are the same. We always write 需要， and find 須要 in some popular classic novels, like: 儒林外史˙第一回：「你在此須要小心，休惹人說不是。」 紅樓夢˙第十四回：「如今請了西府裡璉二奶奶管理內事，倘或他來支取東西，或是說話，我們須要比往日小心些。」

需要是你是自身的一种生理状态，须要则是一种外在的要求和条件。