Since I don't have a layout anymore, running characteristics are meaningless right now to me. However, I do have a loop of Atlas track that I occasionally set up to run trains for the girls. My new Athearn SD40-2s run very well with my Atlas B40-8s, C30-7s and U33C. I don't own any Kato diesels, so I can't compare them to the Athearn models, but the Atlas models are probably closer in running characteristics than the Athearns to Katos. That said, the newer revised Athearn SD40-2s are pretty decent performers.
The thing I've never liked about Kato diesels is the way oversize handrails. The tooling of the Athearn models has been updated to reflect a variety of prototypes from early grilles to corrugated grilles, two different styles of dynamic brake sections, Q fans, snoots and short noses. The tooling has also been refined to today's standards for crispness, and the handrails are pretty darn close to scale size. Granted, you don't get treadplate, pilot details, underframe details, flush-fitting glass and the paint varies from thick and incorrect to crisp and darn-near perfect on the various models. But, they are pretty easy to find usually at a good price. I got all my newer releases at $40 each from Discount Model Trains. That gives me the money to add my own detail parts, which as a N&W, Southern and NS modeler (most of the time anyway), can only be a good thing. I can easily spend as much in detail parts as the base model itself.
But, I'm not totally delusional -- I know they'll never perform like a Kato. However, that's not an issue for me in the forseeable future. If performance outweighs prototype fidelity, go with the Kato. If you can overlook a difference in performance from the typical Kato and Atlas diesels, Athearn's model is a no-brainer.