Price for price even if you throw in another 30% on top of the price of a H105 for example, there will be no difference in temperatures.
Pros or AIO:
Cheap
Fast, easy and simple to install
Does the job well and the most overclocked cpu
Cons:
Not unique
Pros of Custom:
Looks better (if done right)
Choice of coolant like the color

Choice of blocks
Expandable
Lower temperatures (if the correct parts are bought)
Cons:
To be as good as or better than AIO, costly (compared to AIO)
Fiddly to install (if you've never done it before)
...but, fun and you get experience.
Requires maintenance.
Price is personal choice, you pretty much get what you pay for with custom liquid cooling, bigger thicker more expensive radiators = better cooling
Edit: Not sure what CPU you have, if it's anythink after Sandy Intel then you're not going to really benefit from full custom, yeah you'll get lower temps but the majory of the CPUs will hit their max OC due to voltage before you hit max temp with a H105.
Sandy (or below) or Extreme performance Intel or high performance AMD CPUs will greatly benefit from a good custom loop setup as most of the time they hit max temp before you run out of free volts and MHz.