Listen... it all depends on what is in your loop.

If you need head pressure because you have a number of restrictuve water blocks in your loop, then you need a 355 with aftermarket top.

If you have an uncomplicated and un restrictive loop, then you need to get a 655 because it has better flow.

At the end of the day, flow is the measure that's important. However, if you have restriction, then head pressure will come into play because your water cannot flow well. If your water does flow well, then you should maximize flow and not pressure.

As to the reliability issue, its a whole battle, and I'm not going to get into that. I have several of each, and they both have their applications.

No one can tell you which pump serves you better until you tell us how your loop is designed, and with which components.