There is some dependence on flow for radiator performance:
http://www.overclockers.com/articles778/index04.asp
In this case you're definately going to be on the lower end of the flow axis with that weaker pump, 3 blocks and a radiator. Probably though with 2 radiators you'll have enough dissipation either way that coolant temps will be fairly low (unless you've got a TEC or something in that loop). I'd probably try parallel first.