Strictly speaking, 勉強することは大変です can have a nuance of "generally speaking," whereas 日本語を勉強するのは大変です tends to be used for particular things. The difference of formal vs. colloquial might be also form this point.
日本語を勉強することは(一般的に言って)大変です。
日本語を勉強するのは(僕にとって)大変です。
They are basically interchangeable, though.
They are basically interchangeable, though.