The Kracken X40 is pretty good AiO Watercooling unit. The downside to water cooling is that you have to worry more about parts failing/breaking, in the event that it does happen. Also, you have to pay a bit more attention to the airflow in your case, so your VRMs get proper cooling. For performance, it'll definitely cool the CPU better, but will be a degree or two louder.
The ETS-T40 is more of an entry-level cooling solution, roughly on par with a Cooler Master Hyper 212 EVO. It'll be okay for light-to-medium overclocking. Extreme overclocking is where the first generation direct-touch design of its heatpipes will show through Compared to the Kracken, you may consider it, in general, to be more reliable and quieter.
I haven't seen a definitive comparison of the new variants of the ETS-T40 (the Black/White) to justify the extra cost. The bare-bones variant, the ETS-T40-TB, has been on sale for $30, though, and that's a pretty decent price.