I believe in this case your second example would be correct; in the first, you are comparing the subject (expenditure) to a prepositional phrase, which is not the same part of speech. In the second example, "that" acts as a pronoun referencing expenditure.
As an aside, for better flow, I would recommend restructuring this sentence to
Young people's expenditure is more evenly split in Canada than in Poland.
This makes it clear that the subject is being compared to itself, with different modifiers, as opposed to another object, and places the interchangeable parts of speech (in canada/in poland) close to each other.