。
I'd rather go today than tomorrow.
Literal
Tomorrow [more-than-よりも] today go-want [explanatory-んだ].
~よりも marks the basis of comparison — 'more than tomorrow.' 行きたいんだ uses the explanatory んだ to give the preference a reason-giving tone — 'the thing is, I want to go today.' This is a clear demonstration of the ~より comparison structure.