Either is just fine. The pumps make a bit of noise, very small. Some say the 655 is quieter than the 355.
The 655 fixed speed runs at a speed of 4. The vario 655 runs from 1-5 speeds, the vario is more powerful.
The 355 with the right aftermarket top has more flow AND head pressure than the 655. It's also smaller. I run 2 355 with the XSPC restop. They are great.
My annual rebuild log showing pumps etc:http://www.overclockers.com/annual-water-cooling-cleaning-rebuild-journal/
I have used both pumps. The D5 is nice but kinda aquaward, the 355 is the best once you get an afetermarket top.
Here is some links you should keep forever. There is pump testing curves etc so you can see it yourself.http://martin.skinneelabs.com/http://www.skinneelabs.com/