I own bags from both designers and I love them equally but differently. Quality wise, MK bags are just as nice as MJ bags, but their design really set them apart. I personally think MK bags are more conventional and even though some of the styles are "daring", which includes using animal hair or exotic skin and loud color choices, they are still within the boundary of his "style box". I also seem to think his style is more towards European style.
But with MJ bags, even though he always come up with some classic styles, more of his design are "out of the box". His bags are usually very trendy, you've never seen it before but you recognize it as his design because of some "signature" styling (like the padlock), and sometimes he mixes different patterns or materials that others would think shouldn't work, but it actually works. And they are what makes his bags unique and original, but still elegant and classy.
So it all depends on which style you like better...=)