Hello,
I have an Accelero S1 rev 2 and yes, in my opinion it is better to have a fan on with a GTS 250. However I'm using a single 120mm Noctua at 800rpm that is very very silent. Zip-ties were used to fix the fan on the plastic frame
The Accelero TWIN TURBO Pro seems very good. I didn't have any experience with it; maybe you will hear a little bit more noise comparing to my setup.
The Accelero is not difficult to install, but you have to pay a little attention. There are also some little coolers for the VRM modules and RAM chips that are self adhesive, you have to clean well the chips' surfaces before applying them, other way you risk that don't glue enough and they can fall.
In summary:
Accelero S1 (rev 2 or Plus) very good for extreme silence but more bulky (check that it will fit in your case) and you need a fan to be safe.
Accelero TWIN TURBO Pro, silent (I think), very good for cooling, more compact and fans are already installed.
Hope you can choose the best for you.