water cooling: 10mm/12mm or 12mm/16mm which to choose?

Subject says it all. I'm stuck trying to decide on the tubing size. I have to course the tubes will take and I haven't seen any builds, in person. Any suggestions as to which is best would help. Also, this is going to be in a mid-tower cooling the CPU (and maybe GPU in future).Thanks.
 


I haven't. But I've come down to aesthetics. And most lines I know, all but one...CPU block to pump. Otherwise, I know. Just have to decide which size tubing then choose brand/size fittings. I had Bitspower fittings with 10/13 but price was high and want to bring it down.