I'd say they're about equal for me. Thief is of course a timeless classic, I love it to bits, and the fan mission community keeps churning out top notch content. Dishonored is also great, though, and the level design of Dishonored 2 is some of the best level design I've seen in a commercial release ever. So when it comes to level design, the two series are about equal - Clockwork Manor, Grand Palace, etc, are amazing levels, but so are Bonehoard and The Sword.
When we go down to the systems, it comes down to preference I guess. Thief is the purer stealth game, and it's mechanically more solid in its stealth. You have great sound propagation, different surfaces have a different footstep loudness, light and shadow play a huge role in how easy you are to detect and the light gem always unmistakably tells you how obvious or hidden you are.
Dishonored has worse sound propagation, surfaces play less of a role in determining your footstep sounds, there are no major light and shadow mechanics, stealth is mostly line of sight based and you evade guards by hiding somewhere vertical rather than hiding in shadows.
As for the exploration, Thief has rope arrows for vertical movement while Dishonored has blink. Both have plenty of vertical exploration.
If you play without powers, Dishonored is actually harder to stealth than Thief. Guards tend to be a bit more aware and able to turn their heads, while in Thief guards can be cheesed quite easily. Knockouts are quicker in Thief than in Dishonored: the blackjack is instant, choking out takes a few seconds. Some guard patrols in Dishonored, especially Dishonored 2, are designed to require powers or sleep darts to take out. Most guard patrols in the original Thief games can be KO'd one by one with the blackjack. Dishonored also has dynamic patrol routes I think - in the flooded district level in Dishonored 1, I remember that a different guy took over a patrol route after I knocked out the original guard.
Overall, the Dishonored games aren't and don't want to be pure stealth games. Thief is better at that, but yes, the AI isn't up to par with newer games anymore.
I like them both about equally. Though Thief still has the edge simply because of the modding scene. If Dishonored had a map editor, it could surpass Thief at some point. But it doesn't, so it won't.