The jist of it is basically:
Air = More affordable, equivalent performance, quite large (Dependant on the cooler).
AIO Liquid = A bit of a premium over air, more aesthetically pleasing (Most AIOs are pure black all round, ofcourse, this is a matter of opinion), and they have a bit more flexibility in regards to compatibility. (I'll extend on this below).
The general rule when it comes to cooling is that more surface area = better, in other words, the larger something is, the better cooling it will provide. A top end air cooler, such as the NH-D14 is quite a chunky lump of metal, you may have to accommodate accordingly in your case. Think about ram clearance etc...
An AIO (all in one) loop allows for a bit less issues in this department because instead of using a large chunk of metal fins it uses a more compact radiator which is generally much slimmer, the radiator, along with its fans, will then be mounted to an available location inside your case, this is dependant on the size of radiator you choose.
For example, you can get 120mm AIOs that will mount to the rear fan mount on your case, or any other 120mm location. These can come in small, thin radiators, or dual thick, chubby radiators (Such as the H80i in my sig). Moving upwards you can get 240mm radiators, these offer better cooling to their 120mm counterparts and generally mount to the top of the case (However, some cases can support them in multiple locations.).
There is a catch with these, not all cases will have enough space for the 240mm radiator and the fans on it, it may interfere with the motherboard. If you have a relatively good case this shouldn't be an issue.
A negative point to liquid cooling would be the added points of failure. Rare, but when you compare the points of failure on an air cooler, ie, just the fans, to that of an AIO - Fans, pump and additional features the pump may have (Such as LEDs) it's up for concern.
Generally the top end air coolers will trade blows respectively with the top end AIOs, there's not much in it. Either or will serve you well, just have to decide on the pro's and cons.
It starts getting interesting when you move up to 'Proper' water cooling. Not an all in one loop, but a fully custom one. Much, much more expensive and, depending on rad size, better performing. This represents the absolute best, and, being entirely custom gives you the ability to choose tubing/coolant colour/s to match your setup. It takes more maintenance, and it's a bit more lengthy to install.
A custom loop will usually feature a faster pump, and give you the ability to choose a radiator size greater than 240mm, providing your case can accommodate it.
Products to take a look at:
Noctua NH-D14. There is a 15 now, but I don't know if it's available yet. (Not too clued up on air coolers aside from the popular ones, sorry dude).
Corsair H80i or H100i.
Hope this helps.
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