In relation to a random on-board cheap Creative Labs (or equivalent) interface, a dedicated interface makes less noise and jitter in its converter(s). And, like you already stated, the latency is usually WAY lower because of fancy ASIO drivers.
Given that you're mixing ITB, I don't see how pre-amp quality would be relevant.
Try listening to a commercial pop song through an on-board interface and the MOTU alternately, through the same set of speakers/headphones. If you still can't hear any difference, don't waste your money.