Where does the Tuniq Tower 120 come into play?
As far as the Thermaltake BigWater SE, I used one of the All-In-One watercooling kits and wasn't too impressed. In order for it to work effectively, it had to suck in cool air, meaning I had to connect it to the front of the case and the hot air would be blown into the case and left for the other two fans to suck it out. Wasn't very effective.
I'd probably stick with the Tuniq.
Also, if your CPU coolers are doing a good job already, than there is no need to replace them, unless you decide to do some heavy overclocking.