PEX vs Hep2O

[WWilts]

Builder's subbie plumber mentioned Hep2O 15mm
And a bronze pump "secondary return" for quick hot water at kitchen sink, basins

Until then I had assumed it would be PEX pipes 
 

Question: Is there much in it between Hep2O & PEX pipes? Initial cost is a consideration

[MJNewton]

PEX is relatively stiff and so I tend to prefer the flexibility of Hep2O (PB). I've not much experience of the former though so it might not be bad in practice.