I have both.
Most of the time I pay a little more for the AIO cooler because they're easier to manage and the fans add exhaust pressure to the top of the case. I can also spotcool the VRM. Primarily I buy 240mm or 280mm AIO coolers because the 120mm coolers are worthless. There isn't a huge leap from going with a Corsair h115i, EVGA CLC 280mm, or Kraken x62 over the Noctua D15, because the only time those large AIO coolers beat out the large air cooler is when you have the fans at 100%. In the end the difference is an opinion like Coke vs Pepsi or Ford vs. Chevy.
The drawback of the AIO cooler is the radiator has an aluminum core. That and the pump really isn't that fast. They really aren't any better or worse than a comparable heatsink. If you put a high RPM aftermarket fan with a lot of static pressure on the D15, it would outperform any of these AIO coolers with their stock fans.
AIO coolers break all the time and the main thing that goes out on them is the pump. When AIO coolers came out in 2010, there were many complaints out leakage but that doesn't happen quite as much because the manufacturing process is more refined. Corsair for instance, will replace the parts in your PC if one of their AIO coolers leaks and destroys the parts. NZXT offers a limited guarantee that won't cover everything. Aside from that, if you're looking for reliability then go with an aircooler.
Let me put it to you this way. If I was building the system for someone other than myself, I would use an aircooler. Like I said, too much can go wrong with an AIO cooler and when it goes wrong it might happen at the wrong time. AIO coolers are only good if you're trying to manage space in your PC or you're going to overclock. You need a really big AIO cooler for overclocking.
Personally, I haven't had an AIO cooler break and I've owned quite a few of them. Some of them ran for a really long time and they had no trouble. I had a PC paired with the h8 and a Core i7-2600k. The H80 was good for moderate overclocking and it ran with that Core i7-2600k for almost 5 years before I replaced it in favor of the H115i and a Core i7-6800k.
If you're getting a Core i7-8700k then I suggest looking at something that's a step up from the H7 and Hyper 212. Those things run hotter than they should. The Scythe Fuma is a really good pick if you can find one. If you're getting a Core i7-8700, keep in mind that it comes with a stock cooler. CPU temperatures can be somewhat controlled by paying attention to see if the CPU is being overvolted by the BIOS. The BIOS has been known to add more voltage than what's needed.