[icebear]

Regarding the use of a reservoir the pros of it would be that it's a lot easier to get the air out of the water. And perhaps you get lower noise levels from the pump if it's submersible. But then the heat generated by the pump adds to the water.

I don't think that there is any cooling advantages with a reservoir. The water in the reservoir will be heading for ambient temperature, although slow, since a reservoir is a lousy design for air/heat exchange.
If you can get the air out of a closed non-reservoir system it should be just as efficient. If you got a case temperature that is higher than the water temperature, the reservoir even adds a small amount of heat to the water.

I would definitely go for a non-reservoir system, just make sure that you get rid of all air. Less weight, and less space needed for the cooling eq.