Ok - one thing to remember is that the flow-rate at any point in the entire system is always going to be the same. What is going to change is the speed of the water - faster through thinner areas and slower through larger areas. That - and the flow-rate will of course depend on the resistance of the entire system as a whole.
Now - if you use 3/8" tubing you will have a lower flow-rate than if you use 1/2" tubing. You can never have too-much flow-rate, and having lower resistance will make your pump quieter and produce less excess heat. But 1/2" tubing also means you need to change the fittings on your block.
Because the inside channels of your block (regardless of fittings/tubing) are relatively small, the water will flow faster through the block than through the other parts of the system. This should give you a larger temp delta between the block and the water, which should allow the water to keep the block cooler. When the water exits the block and enters into a much larger rad, it will slow down - which gives the rad more time to cool the water down.
So - depending on how much flow you are looking for, larger tubing will give you lower overall resistance in the circuit and a slightly higher flow-rate. With the insides of the block already likely much less than 3/8", I don't know how much difference it would make.
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