I personally say, the lower the better.
My opinion is that even in a closed loop system, the pump will still have to "suck" water into the inlet if it is higher than the lowest point in your system and you dont want your pump "sucking" at all.
If you have a res. near the top of your system and your res. is feeding water to the inlet, than having your pump at the lowest point is a must.
Like I said, this is my opinion. No i dont have any proof other than water wants to travel to the lowest point and stay there
