Whether or not one will fit is hard to say. If you have a 120mm fan in the rear then chances are good.
All in one coolers are better for a few reasons.
1. Liquid and metal conduct heat better then air. Heat is transfered from the copper block to the water, which in turn releases it through the radiator.
2. While they may not fit in all casses, they are less cumbersome when compared to air coolers of the same caliber. This can make the pc much safer to movr around.
3. When compared to smaller coolers, they are often quieter when cooling at the same level. So really they are a great way to get both performance and silence.