I think the larger driver typically offers more output and lower crossover point / lower controlled directivity. This would be different with different drivers but true in general as a concept.
If I'm wrong someone smarter than me will correct me
But using something like a 12" isn't a small driver so the importance of a 15" might be small. Personally I think I'd rather a dual 12" if you wanted to get silly, there is more / better advantages I think.