What is the difference between these two sentences?
They seem very close in meaning.
The difference is,
只要 is used for the only condition, while
如果 usually doesn't.
[ As long as it doesn't rain tomorrow, ... ]
"It doesn't rain tomorrow" is the only one condition, the other things don't matter, e.g.
wind blowing. So you could follow the sentence with
即使刮风我们也去。 (Even it's windy we still will go.)
[ If it doesn't rain tomorrow, ... ]
"It doesn't rain tomorrow" isn't the only condition, that means if it's windy tomorrow, we might not go climbing too.
如果 - if
只要 - as long as
When not used in precision condition, they are very close. But for those of very strict documents, like business contract, legal documents, and academic papers, they are different. 只要 means sufficient condition， while 如果 means a general one.