Water cooling went to 1/2" when water blocks required high flow rates to acheive good cooling (ie, spiral designs).
These days the water blocks that perform best (LRR cascade G4 etc) work on pressure drop to deliver impingement, and work very well at lower flow rates.
Hence, 3/8th tubing is no longer a drama if you're using such blocks, and have "reasonable" tubing runs.
If you're desiring mega flow rates, or are running many meters of tubing to get outside etc then you may consider 1/2"
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