They are more or less similar, but the reason I prefer ES18 has to do with presentation. It has got better separation and gives a spacious feel even within a smaller sound stage. PL11 has a classic 3-blob sound stage (left, right, center) which works well with normal music, but tends to get a bit congested with busier music.
PL11 has better bass punch, crisper and warmer mids, better sparkle in treble. But, ES18 has better bass details while being comparable in bass quantity, forward and clearer mid range, but a slightly rolled off treble. ES18 has a thick and vague mid range, but due to the has a foot tapping nature to it's sound (may be that's what audiophiles call PRAT factor), I prefer it very much over PL11.