Welcome Wendel.
I would like to first point out that we do not get a choice of which to use.
If there is a metal underground water pipe available we must use it to comply with the NEC.
Once we use it it is highly likely to be carrying a few amps of current.
It is very likely that the metal water pipe in one house is electrically continuous with many other houses each with their services bonded to the water pipe.
The entire areas metal water system becomes a conductor for unbalanced neutral current.
Considering the high number of services done this way and the relatively few plumbing problems I do not believe that this current flow is a problem as far as electrolysis.
I think for electrolysis to be a problem you would need a difference of potential. The entire pipe and water would be very close to, if not the same potential.
