It's true that they are similar on their expressiveness approach and at the end core.logic could have an SMT making them identical. But when we add SMT we are talking about a different beast.

For example, imagine that you want to implement a professional chess software. One romantic approach would be to find the best and nicest algorithm(s) to do that. SMT, idealistically, adds some "ugliness" trying to reach to a conclusion using every heuristic available. It's an algorithm too but it doesn't end on a good expressiveness but in inserting every heuristic available inside the resolution process.