Great question! They are similar in that they are both Genesis theme DEV tools and they both were developed by me (which is why you're going to find some similar features scattered throughout), but that's really where the similarities end.
Dynamik is a theme, DevKit is a Plugin.
Dynamik has been around for many years while DevKit is only about a year old.
Dynamik essentially has our Extender Plugin built-in while DevKit does not (but DevKit + Extender costs the same as Dynamik).
Dynamik essentially customizes itself (since it is a Genesis Child Theme), but allows you to export its design as a stand-alone Genesis Child Theme. DevKit on the other hand creates a stand-alone Genesis Child Theme from the start and then allows you to easily customize it. So DevKit can come and go as you need it to, all the while your Child Theme remains fully independent.
Since DevKit was developed from scratch not that long ago its code, UI, and overall functionality is inevitably more current when compared to Dynamik. And that's not a slight against Dynamik (Dynamik has had many facelifts over the years to keep it going strong), but just how software works.
So essentially Genesis DevKit has, in my opinion, a more current, refined, and robust design UI, and it is much more of a pure Genesis Child Theme development tool than Dynamik. Having said that Dynamik has some solid Skin designs to work off of (though I'm working on some pre-customized DevKit Themes as well) and still provides one of the most robust design and customization capabilities out there for Genesis.
When I first released Genesis DevKit it was just an additional Genesis DEV tool in the Cobalt Apps arsenal. But as it has grown as a Plugin and as Dynamik continues to age a bit, I do see it more and more as the successor to Dynamik. And just to be clear, I'm not saying that Dynamik is going anywhere, just that these two tools are less siblings and more parent and child, if that makes sense. DevKit came out of my experience in developing and maintaining Dynamik, and as such it has become more and more of what I see as the future of the "ultimate Genesis theme design tool".
So yes, for those trying to decide between either Dynamik or Genesis DevKit, I would generally recommend DevKit. But again, that doesn't mean that Dynamik can't still get the job done very well. It's just that DevKit, at this point, is the more current tool of the pair.