There is a lot of variable's but 1/2" will not be that much better. I tried 3/16ID all the way up to 1/2"ID and temps didn't change more than 1C on any of the sizes. But that was all on the same pump/rad combo on 3 different blocks of my own.
I like 3/8" and 1/2" more for the ease of use than anything else.