an all in one gets you about 4-5*c cooler then a top end air cooler. A custom kit like the xspc ones will net you around 10-15*c cooler then a top end air cooler. You can also add to the xspc units to include your gpu/s down the road, all in ones cant do that.
I just find the AiO coolers to be that middle ground grey area. Yes they are slightly better then a high end air cooler for the same price, but you still run into pump reliability issues. Or you can go in for more and get the xspc custom cooler that will do a lot better then both, you still can run into pump issues but they seem to be a lot more reliable then the AiO.