The Duncan 59 still sounds like a relatively "modern" pickup to me - immediate attack, slightly compressed, slightly scooped, rolled off high end.
The Mules have more presence, richer mids and harmonics, more clarity and a crunchier attack, for want of a better phrase! I like the 59, it's a very versatile pickup and avoids the honk that a lot of higher output Seymour Duncans have, but the Mule, to me, is the sound of a classic Les Paul distilled.
The Mule splits can be weak but do fundamentally still sound good. I've got into sticking partial splits on my Les Paul and it makes them much more useable.