In terms of overall usability, speed, and stability IDEA is incomparably better. Yes, there are a few minor things that eclipse does better/IDEA can't do (so I keep reading). But this is hugely overshadowed by the fact that IDEA is a high-quality, well-supported professional-feeling product. In comparison, eclipse looks, smells, and acts like a kludgy, bloated, long-lived open source project.
The main important difference is that eclipse is free, and thus some folks seem to really go to bat defending it. There is a certain powerful frugality ethic among developers.
But really, if you're not going to get bent out of shape about spending a few bucks for a piece of software you spend many of your waking hours using, there is no serious comparison.