The only way to know for sure is run some experiments and see. Mitsui may or may not have had to change their disc construction and dye formulations when moving from 12x rated discs to 24x. I don't know. If they did change something, then one or the other of the discs might work better at 4x, and in fact it might well be that the 12x works better at 4x.
Also, some of Mitsui's earliest efforts apparently were not up to the standards of their later media. This is understandably so, since they were still perfecting their new dye and related processes. But I think those problems predate 12x media.
In the final analysis, I think BuddhaTB's answer has the right spirit even if it may not be strictly correct. You will probably see comparable quality with both discs, so the sale price of the 12x becomes an inducement to choose it.